Molecular geometries of H2S

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Molecular geometries of H2S ICF3 and H2O ICF3 characterised by broadband rotational spectroscopyw Downloaded by University of Bristol on 10 April 2012 Published on 24 October 2011 on http://pubs.rsc.org | doi:10.1039/C1CP22339A

Susanna L. Stephens, Nicholas R. Walker* and Anthony C. Legon* Received 18th July 2011, Accepted 27th September 2011 DOI: 10.1039/c1cp22339a The rotational spectra of three isotopologues of H2S ICF3 and four isotopologues of H2O ICF3 are measured from 7–18 GHz by chirped-pulse Fourier transform microwave spectroscopy. The rotational constant, B0, centrifugal distortion constants, DJ and DJK, and nuclear quadrupole coupling constant of 127I, waa(I), are precisely determined for H2S ICF3 and H2O ICF3 by fitting observed transitions to the Hamiltonians appropriate to symmetric tops. The measured rotational constants allow determination of the molecular geometries. The C2 axis of H2O/H2S intersects the C3 axis of the CF3I sub-unit at the oxygen atom. The lengths of halogen bonds identified between iodine and sulphur, r(S I), and iodine and oxygen, r(O I), are determined to be 3.5589(2) A˚ and 3.0517(18) A˚ respectively. The angle, f, between the local C2 axis of the H2S/H2O sub-unit and the C3 axis of CF3I is found to be 93.7(2)1 in H2S ICF3 and 34.4(20)1 in H2O ICF3. The observed symmetric top spectra imply nearly free internal rotation of the C2 axis of the hydrogen sulphide/water unit about the C3 axis of CF3I in each of these complexes. Additional transitions of H216O ICF3, D216O ICF3 and H218O ICF3 can be assigned only using asymmetric top Hamiltonians, suggesting that the effective rigid-rotor fits employed do not completely represent the internal dynamics of H2O ICF3.

Introduction Recent works describe the self-assembly of macroscopic crystal structures from hydrocarbon chains that coordinate through halogen bonding interactions.1,2 The macroscopic properties of these materials are determined by the binding strengths and geometries of the contained halogen bonds. A simple example of a halogen bond is the interaction between a simple Lewis base such as H2O and the iodine atom of a small, halogenated iodoalkane such as CF3I. Gas-phase studies allow measurements to be performed on isolated species and inform understanding of the analogous interactions in condensed phase systems. Microwave spectroscopy provides an opportunity to explore the fundamental nature of halogen bonds precisely and selectively.3,4 The geometries, binding strengths and degree of charge transfer4 have been studied for halogen bonds to Lewis bases that include NH3, H2O, H2S, C2H4 and C2H2. The interaction between oxygen and chlorine or bromine has been explored through studies of H2O XY (where XY = Cl2, ClF, Br2, BrCl or ICl).5–8 These complexes are found to have equilibrium geometries of Cs symmetry where the C2 axis of the water

School of Chemistry, University of Bristol, Bristol, BS8 1TS, UK. E-mail: nick.walker@bristol.ac.uk, a.c.legon@bristol.ac.uk w Electronic supplementary information (ESI) available. See DOI: 10.1039/c1cp22339a

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molecule intersects the axis defined by the collinear O XY atoms to define an angle, f. The magnitude of this angle is related to the orientation of the lone pair on the oxygen atom by empirical rules that predict the molecular geometry. Each complex rapidly inverts between two equivalent configurations where f has equal magnitude but opposite sign so that the geometry is effectively planar on the rotational timescale. For the same range of XY, the geometry of H2S XY is found to be Cs pyramidal with no evidence of the inversion seen in H2O XY. The nature of the bond(s) formed between halogencontaining molecules and water has attracted particular interest because of the opportunity to examine the competition between halogen and hydrogen bonds. The strength of halogen bonding increases on descending group 17. An illustration of the difference between bonding to fluorine and chlorine substituents is provided by recent studies of H2O F4C and H2O ClCF3. The oxygen atom is positioned on a C3 axis of the tetrahedral CF4 unit in the former, equidistant from the three nearest fluorine atoms.9 The fourth fluorine, F(4), is on the opposite side of the central carbon such that the angle, O C–F(4), is 1801. The geometry10 of H2O ClCF3 is partly defined by a halogen bond between the chlorine and oxygen atoms to give a C–Cl O angle of 1801. The fluorines are positioned on the opposite side of the CF3Cl unit and thus, do not interact with the water molecule. Hydrogen bonds determine the geometries of complexes formed between CH2F2 and water. The water molecule orients Phys. Chem. Chem. Phys., 2011, 13, 21093–21101

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so that hydrogen bonds can form between the oxygen atom and CH2F2 while fluorine atoms in the latter bind to the hydrogen atoms in water. Very recent studies11 of H2O HCClF2 and H2O HCBrF2 identify the simultaneous presence of CH O and OH X interactions (where X = Cl or Br) between the monomers. The halogen bond between water and iodine has been less widely investigated. The geometry6 of H2O ICl was found to be Cs pyramidal at equilibrium but the complex undergoes rapid inversion to yield an effective planar structure analogous to that described above for complexes8,12 of H2O BrCl and H2O HCl. The present work explores the nature of the halogen bonds formed between iodine and either oxygen or sulphur in H2O ICF3 and H2S ICF3. The principal objectives of this work are determination of the molecular geometries of H2S ICF3 and H2O ICF3, determination of the nuclear quadrupole coupling constant of iodine in each complex, waa(I), and the force constant of each halogen bond from the measured centrifugal distortion constants. It will be shown that the data are consistent with those of other complexes where the internal dynamics mean that simple Hamiltonians do not reproduce the observed spectra. The spectra of A and E species arising from nearly free internal rotation were a feature of the microwave spectra13 of the C3v symmetric tops, H3N ICF3 and (CH3)3N ICF3. The r(S/O I) bond lengths in H2S ICF3 and H2O ICF3 will be shown to be of similar magnitude to the r(N I) bond lengths in H3N ICF3 and (CH3)3N ICF3. The spectrum14 of H2S C6H6 was interpreted using a Hamiltonian appropriate to a symmetric top. Nearly free internal rotation in this complex causes an effectively symmetric distribution of the mass of the hydrogen atoms of H2S about the C6 axis defined by C6H6, resulting in the transitions characteristic of a symmetric top. Similar interpretations have been applied to the spectra9,10,15–17 of H2O C6H6, H2O F4C and most pertinently, H2O ClCF3. In the last case, m = 0 and m = 1 transitions were observed, although only the former could be completely assigned. Many transitions in the spectra of each of H2S C6H6, H2O C6H6, H2O F4C and H2O ClCF3 could not be accounted for by simple Hamiltonians. It will be shown that symmetric top spectra are observed for all four isotopologues of H2O ICF3 and all three isotopologues of H2S ICF3 studied. Transitions with both m = 0 and m = 1 (where m is an internal rotation quantum number) are observed in the spectra of H216O ICF3 and H218O ICF3. Many unassigned transitions remain in the spectra of H216O ICF3, D216O ICF3 and H218O ICF3 even after those assigned to symmetric tops have been removed. For each of these isotopologues, many of the remaining transitions are now assigned by assuming a Hamiltonian appropriate to an asymmetric top. The implications of this unexpected, simultaneous observation of distinct series of transitions that respectively assign to symmetric or asymmetric top Hamiltonians for H2O ICF3 are discussed.

Experimental A comprehensive description of the chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer is provided in a recent work18 so only a brief description will be provided here. 21094

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An arbitrary waveform generator (Tektronix, AWG7102) is used for the initial generation of chirped microwave pulses that sweep from 0.5–12 GHz over a duration of 1 ms. Each chirped pulse is passed through a low-pass filter (12.2 GHz, Lorch 11LA-12200-s) before mixing against the 19.00 GHz output of a phase-locked dielectric resonant oscillator (Herley CTI, XPDRO-14373). The AWG and PDRO are each phaselocked to a 10 MHz frequency reference (Symmetricom 58533A, GPS time and frequency reference receiver) accurate to 1 part in 1011. The output after mixing (7–18.5 GHz) is amplified prior to irradiation of the molecular sample. The data presented herein were acquired using a 300 W travellingwave-tube amplifier (Amplifier Research, 300IJ) that replaces the 5 W solid-state amplifier used previously.18 The 300 W amplifier permits transitions to be efficiently measured across a 12 GHz bandwidth simultaneously. Experiments involving H2S are performed using a gas sample of B1% CF3I and the same concentration of H2S in a balance of B98% argon at a backing pressure of 1 atm. A few drops of water added to the tube between sample tank and nozzle allow the generation of H2O ICF3 complexes from a gas sample containing B1% CF3I and a balance of 1 atm. argon. Isotopically-enriched samples of D2S, HDS, D2O and HDO are employed to acquire the spectra of D2S ICF3, HDS ICF3, D2O ICF3 and HDO ICF3, respectively. These complexes are formed in the cold environment of a supersonic jet from a pulsed nozzle (Series 9, General Valve). The axis of the supersonic jet is perpendicular to the propagation of each microwave polarisation pulse transmitted from a horn antenna. All microwave pulses are timed to ensure spatial and temporal overlap with the gas pulse. A pin diode limiter and single-pole, single-throw (SPST) pin diode switch protect the low-noise amplifier used in the detection circuit from the high power of the polarisation pulse. The SPST switch is subsequently opened to allow measurement of the molecular free induction decay after each microwave polarisation pulse. Given that the duration of the nozzle pulse (B200 ms) is significantly greater than that of the polarisation pulse (1 ms) and molecular free induction decay (B20 ms), it is possible to polarise molecules in the gas pulse and detect the subsequent free induction decay (FID) repeatedly within the duration of each gas injection pulse. In the present work, four chirped pulses separated by an interval of 25 ms are used to polarise repeatedly the molecules within each gas pulse. The molecular emission signal is detected by a second horn antenna prior to amplification. The FID between 7 and 18 GHz is mixed down against the 19.00 GHz reference signal supplied by the PDRO. Each FID is measured over a 20 ms period following each chirped polarisation pulse. A digital oscilloscope (Tektronix, DPO71254) is used to digitise the molecular emission signal and perform the Fourier transformation that allows the power spectrum of each FID to be displayed. The train of four FIDs detected following every nozzle pulse is averaged in the time domain. At the conclusion of data collection, this train of four time-averaged FIDs is displayed on the oscilloscope. Each of these is then Fourier transformed individually before the resulting power spectra are summed in the frequency domain. The repetition rate of experimental cycles, ultimately limited by the processing speed This journal is

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of the oscilloscope, is set to approximately two hertz. The oscilloscope is phase-locked to the same 10 MHz external frequency reference used by the PDRO and AWG. Observed transitions have a linewidth (full width at half maximum) of B80 kHz.

Results and data analysis


Observations and spectral analysis of H2S ICF3 and H2O ICF3 Fig. 1 displays a 6 GHz section of a broadband (12 GHz) rotational spectrum obtained from a supersonically-expanding sample of H2S, CF3I and argon. Intense transitions around 9 and 12 GHz are assigned to JK00 00 JK0 0 transitions of the CF3I monomer that have been measured previously.18 In addition, new features that consist of many, closely-spaced individual transitions, are observed separated by intervals of B1100 MHz, consistent with the spectrum of either a symmetric or a very nearly symmetric top. These features require the simultaneous presence of both CF3I and H2S within the gas sample. Similar features separated by intervals of B1700 MHz are observed in spectra obtained from a supersonic expansion of CF3I, H216O and argon. The iodine nucleus within CF3I introduces extensive splitting so that hyperfine structure in the J 0 0 ’ J 0 transition of CF3I (evident at B9 GHz in Fig. 1) is spread over B300 MHz. The profiles of the new, broad features in each of the described spectra are consistent with higher JK00 00 JK0 0 transitions split by nuclear quadrupole coupling of an iodine nucleus with the framework rotation of the complex. We now consider whether any simplifying assumptions might usefully inform the choice of Hamiltonians to describe complexes of H2S ICF3 and H2O ICF3. The structure11 of H2O ClCF3 was found to involve a single halogen bond between the chlorine and oxygen atoms rather than one or more hydrogen bond(s). It is known that halogen bonds involving iodine tend to be stronger than those involving the lighter halogen atoms.4 It can therefore be anticipated that the oxygen atom in H2O and the sulphur atom in H2S will each form a halogen bond with iodine in preference to the fluorine atoms. The O XY and S XY angles in H2O XY and H2S XY, respectively (where XY = Cl2, Br2, BrCl, or ICl)5–9,19–23 have been found to be 1801 in a wide range of complexes. A lone pair of electrons on the oxygen of H2O or the sulphur of H2S is observed to align with the axis defined by

Fig. 1 A section of the broadband microwave spectrum of H2S ICF3. The spectrum is shown after 75 000 nozzle pulses. Hyperfine components of the free CF3I monomer are visible around 9.0 and 12.1 GHz. The intensities of many of these are off-scale and therefore not correctly represented above. Features assigned to H2S ICF3 are indicated.

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the XY bond to determine the positions of the hydrogen atoms in such complexes. Given these previous studies, it can initially be assumed that carbon, iodine and oxygen/sulphur will be co-linear so that the angles, C–I O and C–I S are close to 1801. Furthermore, the evidence of the previous studies implies that the C2 axes of H2S and H2O will intersect the C3 axis of CF3I at an angle, 01 o f o 901, in each of H2S ICF3 and H2O ICF3, respectively. All these assumptions imply asymmetric-top geometries for both H2S ICF3 and H2O ICF3 (Fig. 2), regardless of the magnitude of f and the orientation of the hydrogen sulphide and water molecules. In both9,10 H2O ClCF3 and H2O F4C, the oxygen atom of H2O is positioned on the local C3 axis of the CF3Cl/CF4 unit and the local C2 axis of the water sub-unit intersects the C3 axis of CF3Cl/CF4 at the oxygen. Both geometries are formally asymmetric tops only by virtue of the positions of the hydrogen atoms of the water sub-unit. There is a very low barrier to the internal rotational motion that involves precession of the C2 axis of H2O about the C3 axis of CF3Cl/CF4. The hydrogen atoms thus have great freedom to move around the C3 axis in each case. This high mobility of the hydrogen atoms and the relatively high symmetry of the remaining structural elements cause the spectra of each of H2O ClCF3 and H2O F4C to reflect the geometries of symmetric tops. Recent work has shown that there is a very low barrier to internal rotation of NH3 and N(CH3)3 relative to CF3I in the symmetric-top molecules, H3N ICF3 and (CH3)3N ICF3. The implication of all of these previous works is that a low barrier to internal rotation may be present in H2S ICF3 and H2O ICF3 and those patterns typical of the spectra of symmetric-top molecules should be anticipated. Fig. 3 displays an expanded section of the spectrum of H2S ICF3 that is shown in Fig. 1. Hyperfine components are observed with the distribution of intensities that is characteristic of the spectrum of a symmetric top. The most intense components are found near the centre of the JK00 00 JK0 0 transition. Those that are far from the band centre are observed with lower intensity. The rotational constant is first estimated from the separation between adjacent JK00 00 JK0 0 transitions in the spectrum. The extent of the hyperfine effects then allows the nuclear quadrupole coupling constant to be approximated prior to assignment of each hyperfine component. The data are fitted to the Hamiltonian of a symmetric top with a single quadrupolar nucleus using the graphical simulation and fitting program, PGOPHER;24 H = HR 16Q(I): rE(I)

(1)

Fig. 2 The structural model used to interpret the data for H2S ICF3 and H2O ICF3. r(S/O I) and f are defined as shown. The a inertial axis is almost collinear with the axis of the CI bond.


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Fig. 3 An expanded section of the broadband spectrum of H2S ICF3. waa(I) causes the observed distribution of hyperfine components in the JK00 00 JK0 0 ¼ 10 11 transition. The simulation (bottom) uses the determined symmetric-top parameters and given in Table 1.

where HR is the Hamiltonian that describes the overall and internal rotation in a symmetric top molecule. The second term describes the energy of the interaction between the nuclear electric quadrupole moment of iodine and the electric field gradient at the iodine nucleus. This interaction couples J and II and the coupling scheme chosen is J + II = F. The rotational constant, B0, centrifugal distortion constants, DJ, DJK, and nuclear quadrupole coupling constant, waa(I), are accurately determined by the fit. Simulation (Fig. 3, bottom panel) using the values of the parameters thus determined yields excellent agreement with the experimental data. The spectra of CF3I D2S and CF3I HDS were measured and fitted to the same set of parameters (Table 1). The fitted values of DJ and waa(I) are similar for all three isotopologues. The significance of the observed substantial change in magnitude of DJK on isotopic substitution is discussed in the conclusions. An expanded section of the spectrum of H216O ICF3 (H2O, HDO and D2O should be understood to denote

Table 1

Spectroscopic parameters for isotopologues of H2S ICF3

B0/MHz DJ/kHz DJK/kHz waa(I)/MHz N s/kHz

H2S ICF3

HDS ICF3

D2S ICF3

557.57473(81)a 0.12448(26) 2.7250(47) 2166.55(11) 243 10.2

548.524153(84) 0.12350(27) 4.2139(59) 2167.01(12) 197 7.9

539.86533(9) 0.11893(27) 6.21827(58) 2166.5(25) 208 9.4

a

Numbers in parentheses are one standard deviation in units of the last significant figure. N is the number of transitions included in the fit.

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H216O, HD16O and D216O respectively from here forward) is shown in the top panel of Fig. 4. This spectrum exhibits a more complicated pattern of hyperfine structure and intensities than is observed for H2S ICF3 and the distribution of hyperfine components is not as immediately identifiable with a symmetric top. The implication is that the observed details assign to several overlapped spectra. Fortunately, the observed transitions are sufficiently intense and resolved that extensive interpretation is possible. Extending the analysis applied to interpret the spectrum of H2S ICF3, many peaks assign to the spectrum of a symmetric top with a single quadrupolar nucleus. The procedure applied to determine B0, DJ, DJK and waa(I) is the same as that used to fit transitions in the spectrum of H2S ICF3. A standard deviation of 8.4 kHz is obtained in a fit of one hundred and two transitions, consistent with the line width of approximately 80 kHz (Table 2). In the same way, many transitions in the spectra of H218O ICF3 (Table 2), D2O ICF3 and HDO ICF3 (Table 3) also assign to symmetric-top Hamiltonians. The values of the parameters determined for these isotopologues are consistent with those determined for H2O ICF3. The great majority of hyperfine components in the spectrum of HDO ICF3 assign to a symmetric top as described above. In contrast, many transitions in the observed spectra of H2O ICF3, D2O ICF3 and H218O ICF3 remain after those assigned to a symmetric rotor have been removed. The distribution of these additional transitions permits further interpretation. Empirical tests reveal that two distinct series of hyperfine components each comprised of transitions with K = 1 can be fitted to yield two slightly different values of waa(I) for H2O ICF3. It is then found that these values can be averaged to yield an excellent approximation to the value of waa(I)


Fig. 4 A section of the broadband microwave spectrum of H2O ICF3 after 90 000 nozzle pulses. The inset displays an expanded section. The simulated spectrum (bottom) uses the parameters determined for H216O ICF3, including all those obtained by fitting transitions to symmetric or asymmetric top Hamiltonians. The evaluated parameters are given in Tables 2 and 4.

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View Online Table 2 Spectroscopic parameters for m = 0 and m = 1 transitions of H2O ICF3 and H218O ICF3 respectively, obtained using Hamiltonians appropriate to symmetric tops H216O ICF3 (Symmetric top)

m = 1 (K = 1,2)

m=0 a


H218O ICF3

846.82030(19) 0.1431(13) 1.921(16) 2199.275(84) 102 8.4

b

846.72766(41) 0.1112(38) 8.517(68) 2198.92(17) 114 14.4

m=0

m = 1 (K = 0,1)

812.06102(30) 0.1360(20) 1.840(24) 2198.82(16) 108 13.7

811.99237(34)c 0.1029(22) 23.59(24) 2199.37(23) 96 15.0


a Numbers in parentheses are one standard deviation in units of the last significant figure. N is the number of transitions included in the fit. transitions with K = 1 or 2 are fitted. c Only transitions with K = 0 or 1 are fitted.

b

Only

Table 3 Spectroscopic parameters for m = 0 transitions of HDO ICF3 and D2O ICF3 respectively, obtained using Hamiltonians appropriate to symmetric tops (Symmetric top)

HDO ICF3 m=0

D2O ICF3 m=0


823.29900(14)a 0.1385(9) 2.071(11) 2199.208(49) 143 7.4

801.16374(16) 0.1334(10) 1.937(13) 2199.416(50) 172 10.3

a


determined earlier in the symmetric rotor fit. It was thus discovered that these distinct series of K = 1 transitions both assign neatly to the spectrum of a single asymmetric top that has a value of [(B0 + C0)/2] very close to B0 determined in the symmetric top fit. The rotational constants, [(B0 + C0)/2], (B0 C0), centrifugal distortion constant, DJ, and nuclear quadrupole coupling constants, waa(I) and (wbb wcc)(I) are all determined accurately when transitions are fitted using an asymmetric-top Hamiltonian. A comparison between the experimental spectrum and simulations that respectively use the symmetric and asymmetric top parameters is displayed in Fig. 5. In addition to the described series of transitions with K = 1, transitions with K = 2 and K = 0 also assign to an asymmetric top Hamiltonian. However, including transitions with K = 0,1 and 2 in the same fit causes very large residuals. The parameters displayed in the left column of Table 4 are obtained by selecting and fitting only transitions with K = 0 and K = 1. Simultaneously fitting transitions with K = 1 and K = 2 results in a different set of values for [(B0 + C0)/2] and DJK but all other parameters in excellent agreement. The spectrum for D2O ICF3 also contains transitions that assign to the spectrum of an asymmetric top with K = 0, K = 1 and K = 2. The constants obtained by fitting transitions with K = 0 and K = 1 for D2O ICF3 are also shown in Table 4. Fits of transitions assigned with K = 2 for each of H2O ICF3 and D2O ICF3 are included in the supplementary data.w Transitions in the spectrum of H218O ICF3 assign to K = 1 transitions of an asymmetric top but other values of K are not identified or assigned for this isotopologue. The spectrum of HDO ICF3 contains many fewer transitions than the This journal is

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Fig. 5 The inset of Fig. 4, displaying an expanded section of the broadband spectrum of H2O ICF3 is given again above. The Asym. trace displays a simulation that employs an asymmetric top Hamiltonian and the parameters for H2O ICF3 in Table 4. The Sym. trace is a simulation that uses the Hamiltonian of a symmetric top and the parameters for H2O ICF3 given in Table 2. The Total Sim. trace combines the Sym. and Asym. traces into a single simulation of the experimental spectrum.

Table 4 Spectroscopic parameters for H2O ICF3, D2O ICF3 and H218O ICF3 respectively, obtained using Hamiltonians appropriate to asymmetric tops (Asymmetric top) B0 þC0 =MHz 2 (B0 C0)/MHz DJ/kHz DJK/kHz waa(I)/MHz (wbbwcc)(I)/MHz N s/kHz

H216O ICF3 K = 0,1

D216O ICF3 K = 0,1

H218O ICF3 K=1

846.74496(43)a

801.16414(20)

812.00566(27)

1.95945(58) 0.1318(44) 11.36(26) 2199.34(19) 20.05(48) 62 11.7

3.29610(33) 0.1336(12) 61.71(12) 2200.32(12) 20.20(33) 93 8.3

1.79942(34) 0.1352(18) — 2199.26(21) 20.43(44) 66 10.5

a


spectra of H2O ICF3, D2O ICF3 and H218O ICF3 and the great majority of these can be assigned using a symmetric top Hamiltonian. Asymmetric top spectra are not identified or assigned for this isotopologue. The values of waa(I) determined in the asymmetric top fits for H2O ICF3, D2O ICF3 and H218O ICF3 are consistent with those determined in the symmetric top fits described earlier. The differences between Phys. Chem. Chem. Phys., 2011, 13, 21093–21101

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B0 (Table 2) and [(B0 + C0)/2] (Table 4) in the respective fits for H2O ICF3, D2O ICF3 and H218O ICF3 are all less than B0.07 MHz. The values of DJK (symmetric rotor) and DJK (asymmetric rotor) for H2O ICF3 have opposite sign and vary considerably. DJK changes significantly as a function of K for the fitted transitions. The consistent values of waa(I) and the similarity in the values of B0 (symmetric rotor) and [(B0 + C0)/2] (asymmetric rotor) suggest that all the various symmetric and asymmetric top spectra described above should be attributed to H2O ICF3. This work will therefore proceed on the assumption, to be reviewed later, that the spectra described in this and the preceding paragraph all assign to internal rotation and/or tunnelling states of H2O ICF3. A number of transitions remain unassigned in the spectra of H2O ICF3 and H218O ICF3 even after the initial symmetric top and the subsequent asymmetric top fits described above. The previous studies9,10,14–17 of H2S C6H6, H2O C6H6, H2O F4C and H2O ClCF3 have reported low barriers to internal rotation and/or tunnelling that have allowed the observation of m = 1 transitions in each of these complexes. Spectra in these states have consistently proved challenging to interpret. For example,14 the fit of an internal rotor or tunnelling excited state of H2S C6H6 yielded a negative DJ. The standard deviation obtained when fitting transitions with m = 1 was greater than that achieved by fitting transitions with m = 0. The frequencies of transitions assigned with m = 1 in the symmetric top spectra9,10 of H2O F4C and H2O ClCF3 were not described by a simple Hamiltonian. It is therefore reasonable to expect that transitions arising from excited internal rotor or tunnelling states of H2S ICF3 or H2O ICF3 may be observed, and that such transitions might not fit to simple Hamiltonians. Indeed, some transitions in the spectra of both H2O ICF3 and H218O ICF3 can be assigned to the spectrum of another vibrational or internal rotational state by using a symmetric-top Hamiltonian to yield precise values of waa(I). These new transitions are tentatively assigned with m = 1 while those symmetric top transitions assigned earlier (as described above) are tentatively attributed as m = 0. It is not possible to explore the symmetry, and hence the correct labelling, of these states any further given the available data. The rotational constant associated with the m = 1 transitions is slightly lower (by B0.1 MHz) than that associated with the m = 0 transitions (Table 2). The distortion constants, DJ and DJK, are somewhat different in the two fits. Only one state is observed in each of the spectra of HDO ICF3 and D2O ICF3. This is tentatively assigned with m = 0 because the associated centrifugal distortion constants are similar to those determined for transitions with m = 0 for H2O ICF3 and H218O ICF3. Recent studies13 of the C3v symmetric tops, H3N ICF3 and (CH3)3N ICF3, identified A and E species transitions consistent with a low barrier to internal rotation. These works established values of the centrifugal distortion parameters, DJm and DJKm, that are included in the Hamiltonian provided by Fraser et al.25 to describe the effects of internal rotation in a C3v symmetric top. A sophisticated interpretation of the vibrations and internal dynamics in H2O ICF3 would be needed to understand the physical origin of the various intermeshed spectra observed here. Given significant variation in the values of DJK for the different 21098


isotopologues of H2O ICF3 presented herein, the molecule is probably not accurately represented by the simple Hamiltonian provided by Fraser et al. For this reason, the data are not interpreted to obtain a value for either DJm or DJKm. Structural parameters The model geometry shown in Fig. 2 is consistent with the structure determined recently10 for H2O ClCF3. It is assumed that the C2 axis of H2S/H2O intersects the C3 axis of CF3I to define an angle, 01 o f o 901, and that there is a low barrier to precession of the C2 axis of the hydrogen sulphide/water unit about the C3 axis defined by CF3I. Each of these assumptions can now be informed by insight from the spectroscopic data. Exchanging a single hydrogen atom of either H2S ICF3 or H2O ICF3 for a deuterium atom alters B0 by a very similar amount to the change observed when replacing HDS/HDO with D2S/D2O. These observations suggest that the hydrogen atoms are symmetrically equivalent in both H2S ICF3 and H2O ICF3. The symmetric top spectra of H2S ICF3 and H2O ICF3 are thus consistent with geometries in which the sulphur/oxygen atom is positioned on the C3 axis of the CF3I molecule. The asymmetric top fits for H2O ICF3 yield values of [(B0 + C0)/2] and (B0 C0). The former is extremely close to the value of B0 determined in the symmetric top fit while the latter is only 1.95945(58) MHz. Significant displacement of the oxygen atom away from the C3 axis defined by CF3I would result in a larger value of (B0 C0). The observed (B0 C0) is thus consistent with an oxygen atom that is positioned on the C3 axis defined by CF3I. A small degree of inertial asymmetry is introduced by the position of the hydrogen atoms. The same conclusion also applies to the data for D2O ICF3 and H218O ICF3. It is beyond the scope of this work to explore the internal dynamics that yield the observed spectra. Fortunately, quantitative details of the molecular structure can be obtained by fitting parameters to the measured values of rotational constants for different isotopologues of H2S ICF3 and H2O ICF3. For both these complexes, the rotational constants alone do not allow a distinction between the model geometry shown in Fig. 2 and another possible arrangement that is qualitatively similar to that found in CF4 H2O. However, the correct assignment can be established through examination of the measured values of waa(I). It was shown elsewhere26,27 by means of ab initio calculations that in the equilibrium conformation of the linear molecule, OC Bri Bro, the magnitudes of the electric field gradients at the inner and outer nuclei Bri and Bro (and therefore those of the Br nuclear quadrupole coupling constants) are increased and decreased, respectively, relative to free Br2 by almost equal amounts. The observed zero-point coupling constants, waa(I), are greater in magnitude than w0(I) (where w0(I) is the value18 of waa(I) in free CF3I) in both H2S ICF3 and H2O ICF3. This observation proves that the electric field gradient at I increases in magnitude as a result of the presence of H2S/H2O. Since angular oscillation of the CF3I sub-unit can only decrease the magnitude of waa(I) relative to w0(I), it also shows that I is contiguous with the B subunit and hence that an iodine halogen bond is being observed. This journal is

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Given the qualitative conclusions described above, the remainder of the geometry of the model structure can be defined as shown in Fig. 2. The length of the halogen bond between iodine and sulphur/oxygen is represented by r(S/O I). The angle, f, is defined between the C2 axis of the H2S/H2O unit and the C3 axis of CF3I. The values of B0 determined by fitting the spectra of H2S ICF3, HDS ICF3 and D2S ICF3 allow both r(S I) and f to be determined through an r0 structural fit of these two parameters using Kisiel’s program STRFIT28 (Table 5). The former is found to be 3.5589(2) A˚ and the latter is 93.7(2)1. The data available from the fits of symmetric top spectra for H2O ICF3, H218O ICF3, HDO ICF3 and D2O ICF3 also allow determination of r(O I) and f. The results are 3.0517(18) A˚ and 34.4(20)1. It is possible to make an alternative determination of the molecular geometry of H2O ICF3 using the values of [(B0 + C0)/2] and (B0 C0) established by fitting the asymmetric top spectra of H2O ICF3, D2O ICF3 and H218O ICF3. The values of r(O I) and f obtained by fitting these parameters are 3.0526(15) A˚ and f = 35.3(17)1. The symmetric and asymmetric top spectra tentatively assigned to H2O ICF3 are thus mutually consistent with respect to a single molecular geometry, validating the assumption made earlier that these spectra each assign to different states of H2O ICF3. It is important to note that the assumptions of the above r0 structural determinations make no account of the internal dynamics in these complexes which are not expected to be rigid. An expression provided by Fraser et al.25 was applied to account for the contributions of large amplitude motions to the geometries13 of H3N ICF3 and (CH3)3N ICF3. It is not possible to directly apply this model to quantify the oscillations of H2O or H2S. The variation of r(N I) in H3N ICF3 as a function of the angular oscillation of NH3 about its centre of mass was determined to be B0.001 A˚ while assuming oscillation angles ranging from 0–301. The angle is defined between the C3 axis of NH3 and a line connecting the centres of mass of CF3I and NH3 in this complex. Like NH3 in H3N ICF3, the masses of H2O and H2S are very low compared with that of CF3I. As a consequence, accounting for large amplitude motions (4201) of H2O and H2S would not lead to results that are significantly different from the

Table 5 Evaluated structural parameters, force constants and vibrational wavenumbers for H2S ICF3 and H2O ICF3

r(S/O I)/A˚ f/1 ks/N m1 o/cm1

H2S ICF3 (sym.)

H2O ICF3 (sym.)

H2O ICF3 (asym.)

3.5589(2)a 93.7(2) 6.7 [6.6,6.8]b 63 [62,61]b

3.0517(18) 34.4 (20) 8.1(1) [8.4(1), 8.6(1)]c 91(1) [91(1), 90(1)]c

3.0527(15) 35.3(2) 8.8(3) [8.6(2)]d 95(3) [94(2)]

a

Numbers in parentheses are one standard deviation in units of the last significant figure. Where no standard deviation is given, it is significantly less than the magnitude of the final significant figure. b Numbers in square brackets are the results for HDS ICF3 and D2S ICF3 respectively. c Numbers in square brackets are the results for HDO ICF3 and D2O ICF3 respectively. d Number in square brackets is the result for D2O ICF3.

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geometries determined by the r0 structure fits. A recent study of the microwave spectrum29 of Kr ICF3 establishes that the CF3I sub-unit oscillates through an angle defined by cos1hcos2 fi1/2 = 5.0(5)1. This represents an upper limit to the angular motion of CF3I in the present study given that H2O and H2S are both likely to bind to CF3I more strongly than to Kr. It was shown that an oscillation of 51 by CF3I in H3N ICF3 is consistent with a bond length that is longer than the r0 result by B0.01 A˚. It is worth noting that, even in the absence of large amplitude motion, the geometry determined for H2S ICF3 should be less accurate than that obtained for H2O ICF3 because in the former case only H/D substitutions were possible while for the latter a heavy atom substitution at O was also made. This is apparently not so here, but of course the quoted error in the parameters obtained reflects the quality of the fit and this can be misleadingly good when only a few moments of inertia are fitted, as is the case for H2S ICF3. Bond stretching force constants If it is assumed that the measured centrifugal distortion constant, DJ or DJ, depends solely on the stretching of the weak bond between the two monomer units, a force constant can be calculated for this interaction. The simplest assumption is to treat the molecule as a pseudodiatomic where two point masses (i.e. representing the component monomer units) are separated by a single bond. The results of calculations of the force constant and vibrational wavenumber of H2S ICF3 under this assumption are 10.6 N m1 and 79 cm1. The uncertainty in these results derives from the evaluated centrifugal distortion constants and is significantly less than the final significant figure quoted. The results for H2O ICF3 are 18.4(2) N m1 and 137(1) cm1. Millen30 has provided expressions that take account of the inertial properties of the sub-units in the calculation of force constants and vibrational wavenumbers. This analysis uses the quadratic approximation for force constants so that only vibrations of the same symmetry can contribute to DJ. It is also assumed that other vibrations of this symmetry are too high in wavenumber to contribute significantly to DJ. The expression provided by Millen for the force constant of an asymmetric top containing a hydrogen or halogen bond perpendicular to the plane of a monomer unit is; ks ¼

8p2 mD 3 3 ½BD ð1 bÞ þ CD ð1 cÞ DJ

ð2Þ

where BD and CD are the rotational constants of the complex and mD is the reduced mass. In the example of H2S ICF3, mH S mCF I mD ¼ mH 2S þmCF3 I . The value for b is given by b ¼ BBHDS þ BBCFD I 2

3

2

3

where BH2S is the B rotational constant of free H2S and BCF3I is the rotational constant for CF3I. An analogous expression is used to calculate c from the C rotational constants. The results obtained when using the measured rotational and centrifugal distortion constants of H2S ICF3 and H2O ICF3 in eqn (2) are shown in Table 5. The data available from all isotopologues are in good agreement. The values of ks determined in the symmetric and asymmetric top fits for H2O ICF3 are not the same to within the summed experimental uncertainties, reflecting Phys. Chem. Chem. Phys., 2011, 13, 21093–21101

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the difference in the centrifugal distortion constants determined by the two fits. Extending the definition provided in this work to H2O XY complexes more generally, f represents the angle between the C2 axis of H2O and the symmetry axis of XY. A low barrier to inversion between two structures where f has equal magnitude but opposite sign is a common feature in H2O XY and Millen provides a slightly different expression appropriate to such complexes. Eqn (2) gives a result for ks of H2O ICF3, displayed in Table 5, which is not significantly different to that obtained under the alternative formulation. The possibility of inversion that is rapid on the timescale of the molecular rotation of H2O ICF3 is not explored by this work. The force constants so calculated13 for H3N ICF3 and (CH3)3N ICF3 are 11.6(2) N m1 and 22.7(2) N m1 respectively, suggesting that the binding strengths in these complexes are in the order E(H2S ICF3) o E(H2O ICF3) o E(H3N ICF3) o E((CH3)3N ICF3).

Conclusions The results obtained during this work for the lengths of the halogen bonds, r(S/O I), angular geometries, f, and force constants, ks, in H2S ICF3 and H2O ICF3 are compared with the same quantities for various related complexes in Table 6. H3N ICF3 and (CH3)3N ICF3 complexes were the subject of a recent publication13 and are also included in the table. The halogen bonds formed by iodine are generally stronger than those formed by the lighter halogen atoms and these are stronger when the atom is attached to an electronwithdrawing group such as CF3. The resulting strong interactions are exploited in solid-state applications where perfluorinated alkanes are an important class of building blocks.1,2 This work provides an opportunity to examine directly how the chemical environment at the iodine atom affects the strength of the halogen bond formed by this atom. The most immediate comparisons are possible with the series of complexes6,31,32 formed between H2S, H2O and NH3 with the dihalogen molecule, ICl. The force constants calculated for complexes of the dihalogen are higher, on average, than those determined for the various

Table 6 Bond lengths and angles in halogen-bonded complexes containing H2S, H2O, NH3 and N(CH3)3 (see text) r(B I) a

f

ks b

H2S ICF3 H2O ICF3 H3N ICF3 a (CH3)3N ICF3

3.559 3.052(2)c 3.039(1) 2.781(2)

93.7(2) 34.4(20)c — —

6.7 8.1(1)c 11.6(2) 22.7(2)

d

H2S ICl H2O ICl H3N ICl

3.154(3) 2.838(3) 2.711(2)

91.9(12) 46(2) —

16.6(3) 15.9(2) 30.4(3)

H2S BrCl H2O BrCl

3.094(6) 2.781

96.0(13) 48.0(2)

12.0(2) 12.1(1)

a

a

d d e e

a This work and ref. 13. b Numbers in parentheses are one standard deviation in units of the last significant figure. Where no standard deviation is given, it is significantly less than the magnitude of the final significant figure. c The values shown are those determined from the symmetric top fits (Table 5). d Ref. 6, 31 and 32. e Ref. 8 and 23.

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CF3I complexes. The length of the halogen bond in B ICl (where B = H2S, H2O, NH3) is shorter than the equivalent bond in B ICF3 for H2S, H2O and NH3. These observations imply that the halogen bonding interaction in B ICl is stronger than that in B ICF3. Despite this stronger interaction, the angular geometries of B ICl complexes are very similar to those in B ICF3 for B = H2S or H2O. Exchanging H2O for H2S does not significantly change ks in either B ICl or B BrCl and the values of ks for H2S ICF3 and H2O ICF3 are quite similar. The assumption that DJ is determined solely by stretching of the intermolecular bond is not necessarily reliable in these complexes where large amplitude motions are expected. It is therefore not possible to conclude unambiguously whether H2S ICF3 or H2O ICF3 is the more strongly bound from these results. The symmetry axis of ICl or CF3I is almost perpendicular to the C2 axis of H2S in complexes formed between these monomer units. This angle, f, is much smaller and near to 351 in complexes containing H2O. The same observations apply to structural parameters8,23 in H2S BrCl and H2O BrCl which are also presented in Table 6. Evidently, the rules that determine angular geometries in these complexes are largely independent of the strength of the interaction between the monomer units in each case. Examination of a wide range of halogen bonded complexes and also33–35 H2O MCl, H2S MCl, and C2H4 MCl (where M = Cu or Ag) reveals that this conclusion can be broadly applied to describe the angular geometries of a diverse range of complexes. The spectra of the complexes examined during this work contain evidence for internal dynamics that will require further interpretation by other experimental and theoretical methods. The centrifugal distortion constant, DJK, of H2S ICF3 changes considerably between the different isotopologues studied. The same parameter is also highly variable in the data for H2O ICF3 where the parameter determined by fitting m = 1 transitions of H216O ICF3 is a factor of four greater than that determined by fitting m = 0 transitions of the same isotopologue. The data acquired from different K states in the asymmetric top fits also imply that the selected Hamiltonian may not contain all necessary terms, particularly with respect to DJK. It was not possible to fit transitions with all values of K simultaneously to yield a single value for DJK for either H216O ICF3 or D216O ICF3. Only transitions with K = 1 could be identified in the asymmetric top fit for H218O ICF3. In the same way that DJ (or DJ) can be related to bond force constants and stretching vibrations, DJK (or DJK) is connected with bending and torsional motions. Complicated internal dynamics combining internal rotation and tunnelling effects have been reported9–10,14–17 in H2S C6H6, H2O C6H6, H2O F4C and H2O ClCF3. It is suggested that the anomalous values of DJK and DJK reported herein arise from dynamical effects in these complexes that are not completely described by the Hamiltonians employed. It is proposed that these dynamical effects are further manifested in the spectrum of H2O ICF3 where some transitions are readily assigned using a symmetric top Hamiltonian while many others assign to the Hamiltonian of an asymmetric top. The potential energy surface, internal rotational motions and tunnelling in these complexes are worthy of further examination using high level theory. This journal is

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Acknowledgements The authors thank the Engineering and Physical Sciences Research Council (UK) for a postgraduate studentship awarded to SLS and project funding (EP/G026424/1). NRW thanks the Royal Society for a University Research Fellowship. ACL thanks the Leverhulme Trust for an Emeritus Fellowship and the University of Bristol for a Senior Research Fellowship.


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