Do corresponding coupling constants in hydrogen-bonded homo- and

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Do corresponding coupling constants in hydrogen-bonded homo- and hetero-chiral dimers differ?

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Janet E. Del Bene, Ibon Alkorta, and Jose´ Elguero

Abstract: Ab initio equation-of-motion coupled cluster singles and doubles (EOM–CCSD) calculations have been carried out to evaluate spin–spin coupling constants in six pairs of homo- and hetero-chiral dimers: (HOOH)2, (H2NNH2)2, (FOOH)2, (FHNNH2)2, (HOOOH)2, and (FOOOH)2. Corresponding spin–spin coupling constants in these isomeric pairs of C2 and Ci symmetry may differ, but these differences are small and may not be detectable experimentally. For the complexes with O1–HO and O1–HF hydrogen bonds, 1J(O1–H) has a larger absolute value in the C2 isomer. For the same set of complexes, 1J(O1–O2) has a larger absolute value in the Ci isomer. No distinguishable patterns could be discerned in the remaining spin–spin coupling constants in the C2 and Ci isomers of these complexes, nor in complexes with N–HN hydrogen bonds. Key words: spin–spin coupling constants, homo- and hetero-chiral dimers, hydrogen bond. Re´sume´ : Des calculs ab initio EOM–CCSD ont e´te´ effectue´s pour e´valuer les constantes de couplage spin–spin dans six paires de dime`res homo- et heterochiral : (HOOH)2, (H2NNH2)2, (FOOH)2, (FHNNH2)2, (HOOOH)2, et (FOOOH)2. Les correspondantes constantes de couplage spin–spin dans les paires d’isome`res sont, en geńe´ral, diffe´rentes mais les diffe´rences sont faibles au point, pour certaines d’entre elles, de ne pas pouvoir eˆtre de´tecteés expe´rimentalement. Pour les complexes avec des liaisons hydroge`ne O–HO et O–HF, 1J(O1–H) a une plus large valeur absolue dans l’isome`re C2. Pour le meˆme ensemble des complexes, 1J(O1–O2) a une plus large valeur absolue dans l’isome`re Ci. On ne trouve pas des re`gles geńe´rales pour les restantes constantes de couplage spin–spin dans les isome`res C2 et Ci de ces complexes et non plus dans les complexes avec des liaisons hydroge`ne N–HN. Mots-cle´s : constantes de couplage spin–spin, homo- et hetero-chiral dime`res, liaisons hydroge`ne.

Introduction Chiral distinction or ‘‘chiral recognition’’ has been a topic of interest to both theorists and experimentalists. In the past, we have devoted several papers to the topic of chiral distinction in which we focused primarily on energy differences between enantiomers.1 Experimental NMR studies of nuclear shielding have been reported and used to characterize and analyze chiral discrimination in complexes formed by a molecule of known chirality and a mixture of enantiomeric compounds.2–4 In the present paper, we ask for the first time whether or not corresponding spin–spin coupling constants for hydrogen-bonded hetero- and homo-chiral dimers differ, and if so, do patterns exist that would distinguish one isomer from the other as a tool for chiral distinction.5,6 To answer this question, we have carried out ab initio equation-of-motion coupled cluster singles and doubles (EOM–CCSD) calculations to evaluate spin–spin coupling constants for the C2 (homochiral; optically active) and Ci

(heterochiral; optically inactive) isomers of six hydrogenbonded dimers: (HOOH)2, (H2NNH2)2, (FOOH)2, (FHNNH2)2, (HOOOH)2, and (FOOOH)2. In this paper, we present the results of these calculations, and our conclusions about relationships between corresponding spin–spin coupling constants in the isomeric pairs.

Methods The structures of the monomers and complexes were optimized at second-order Møller–Plesset perturbation theory (MP2)7–10 with the 6-311++G(d,p) basis set.11–13 Frequencies were computed to insure that each isomer corresponds to a local minimum on its potential surface. The geometries of some of the systems have been reported by us in a previous article.14 Coupling constants were evaluated using the equation-of-motion coupled cluster singles and doubles (EOM–CCSD) method in the CI (configuration interaction)-like approximation15,16 with all electrons correlated. For these

Received 1 October 2009. Accepted 30 November 2009. Published on the NRC Research Press Web site at canjchem.nrc.ca on 12 March 2010. This article is part of a Special Issue dedicated to Professor R. J. Boyd. This paper is dedicated to our friend and colleague Russell Boyd. J.E. Del Bene.1 Department of Chemistry, Youngstown State University, Youngstown, Ohio 44555, USA. I. Alkorta and J. Elguero. Instituto de Quı´mica Me´dica, CSIC (Consejo Superior de Investigaciones Cientı´ficas), Juan de la Cierva, 3, E-28006 Madrid, Spain. 1Corresponding

author (e-mail: [email protected]).

Can. J. Chem. 88: 694–699 (2010)

doi:10.1139/V09-177

Published by NRC Research Press


Del Bene et al.

calculations, the Ahlrichs and co-workers17 qzp basis set was placed on 15N, 17O, and 19F, and the qz2p basis set was placed on the hydrogen-bonded 1H. All other H atoms were described using the Dunning18 cc-pVDZ basis set, and no coupling constants are reported for these hydrogens. The EOM–CCSD method with the Ahlrichs (qzp,qz2p) basis set has been shown to yield coupling constants in good agreement with experimental values for molecules.19–22 Its application to hydrogen-bonded complexes has provided insight into experimentally determined coupling constants,23 and the predicted relationship between 2hJ(N–N) and the N–N distance for N–HN hydrogen bonds24 has been verified experimentally.25 Finally, the computed signs and magnitudes of 2hJ(X–Y), 1hJ(H–Y), and 1J(X–H) for X–HY hydrogen bonds have been used successfully to characterize hydrogen-bond type.26,27 In the Ramsey approximation, the total coupling constant (J) is a sum of four contributions: the paramagnetic spin-orbit (PSO), diamagnetic spin–orbit (DSO), Fermi contact (FC), and spin–dipole (SD).28 All terms have been computed for all molecules. The MP2 calculations were carried out using the Gaussian 03 suite of programs29 on the computers at the CSIC (Consejo Superior de Investigaciones Cientı´ficas). EOM–CCSD calculations were done with ACES II30 on the IBM Cluster 1350 (Glenn) at the Ohio Supercomputer Center.

Results and discussion Structures and binding energies Table 1 presents the hydrogen-bonding coordinates R(X–Y), R(X–H), R(HY), and the angle H–X–Y for the C2 and Ci isomers of the six dimers with X–HY hydrogen bonds, and the binding energies of these complexes. The complexes (HOOH)2 and (HOOOH)2 are stabilized by O–HO hydro˚. gen bonds, with O–O distances varying from 2.814 to 2.841 A The (FOOH)2 and (FOOOH)2 dimers are stabilized by O–HF hydrogen bonds. The O–F distances in the C2 and Ci struc˚ , respectively. The tures of (FOOH)2 are 2.811 and 2.755 A corresponding distances are much longer in (FOOOH)2, at ˚ , respectively. (H2NNH2)2 and 3.616 and 3.031 A (FHNNH2)2 are stabilized by N–HN hydrogen bonds, with ˚ . Except for the N–N distances between 3.084 and 3.146 A O–F distances in (FOOOH)2, these intermolecular distances are not unusual. However, the hydrogen bonds in these dimers are not linear, and in some cases the deviation from linearity is significant, as evident from Table 1. The dimers with hydrogen bonds that exhibit the smallest deviation from linearity are the isomers of (HOOOH)2, for which the H–O–O angle is approximately 108. It is also interesting to note that the C2 and Ci isomers of this dimer have the greatest binding energies of 10.9 and 11.2 kcal mol–1, respectively. The largest deviation from linearity is 408 in the Ci isomer of (FOOOH)2. Although this isomer has a shorter O–F distance than the C2 isomer, it is less stable by 1.2 kcal mol–1. The large deviation from linearity makes it debatable as to whether the Ci isomer can even be described as hydrogen bonded, since it may gain stability from electrostatic interactions, including an antiparallel alignment of O–H bond dipole moments, as shown in Fig. 1. It is noteworthy that the

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(FOOOH)2 dimer is the only dimer for which the C2 isomer is more stable than Ci. The other dimers that have significantly nonlinear hydrogen bonds are the isomers of (FHNNH2)2. These dimers have the F atom bonded to the proton donor N1–H, with binding energies of 5.8 and 6.6 kcal mol–1 for the C2 and Ci structures, respectively. These isomers have the weakest hydrogen bonds, except for the Ci isomer of (FOOOH)2. It might have been anticipated that substituting F on the donor N1–H would make HFNNH2 a stronger proton donor, and (FHNNH2)2 would be more stable than (H2NNH2)2, but this is obviously not the case since the (H2NNH2)2 isomers have binding energies of 7.0 and 7.5 kcal mol–1. The reduced stabilities of the (FHNNH2)2 dimers may reflect the significant nonlinearity of the hydrogen bonds in the C2 and Ci isomers, which have H–N–N angles of 328 and 358, respectively. Spin–spin coupling constants In the Introduction to this paper we asked the question whether or not corresponding spin–spin coupling constants for hydrogen-bonded hetero- and homo-chiral dimers are different, and if so, do patterns exist that would distinguish one isomer from the other. The one- and two-bond coupling constants 1J(X–H), 1hJ(H–Y), and 2hJ(X–Y) across the X–HY hydrogen bonds are reported in Table 2. The two-bond coupling constants 2hJ(X–Y) are very small, ranging from 0.42 Hz in the Ci isomer of (HOOH)2 to 3.09 Hz in the C2 isomer of (FOOH)2. However, when comparing coupling constants involving different atoms, it is the reduced coupling constants that should be used to eliminate the dependence of the coupling constant on the magnetogyric ratios of the coupled atoms. The reduced coupling constants are given in Table 3. From Table 3 it can be seen that the reduced two-bond coupling constants for the dimers with N–HN hydrogen bonds are significantly greater than the reduced two-bond coupling constants across O–HO and O–HF hydrogen bonds. Moreover, although the reduced two-bond coupling constants for O–HO and N–HN hydrogen bonds are positive, the reduced two-bond coupling constants across the O–HF hydrogen bonds are negative, and like the two-bond F–F coupling constant for (HF)2, are exceptions to the generalization that reduced two-bond coupling constants across hydrogen bonds are positive.31 There is no correlation between the two-bond coupling constant 2hJ(X–Y) and the corresponding X–Y distance, most probably due in part to the varying degree of nonlinearity of the hydrogen bonds. Although for each dimer 2hJ(X–Y) values are different in the C and C isomers, there 2 i is no recognizable pattern to these differences. For example, 2hJ(O–O) is greater for the C isomer of (HOOH) , but it is 2 2 greater for the Ci isomer of (HOOOH)2. It might be tempting to suggest that this difference may be related to the number of bonds between the proton-donor and the protonacceptor oxygen atoms (1 vs 2). However, 2hJ(O–F) is also greater for the C2 isomer of (FOOH)2 and the Ci isomer of (FOOOH)2, in which case there are two and three bonds, respectively, between the proton-donor and the proton-acceptor atoms. With such relationships, there are too few cases for generalization. From Table 2 it may also be seen that the one-bond X–H Published by NRC Research Press

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˚ ), H–X–Y angles (8), and binding energies (kcal mol–1) of hydrogenTable 1. Distances (A bonded hetero- and homo-chiral dimers. Complex (HOOH)2 O–HO Monomer (HOOOH)2 O–HO Monomer (FOOH)2 O–HF Monomer (FOOOH)2 O–HF Monomer (H2NNH2)2 N–HN Monomer (FHNNH2)2c N–HN Monomer

Symmetry C2 Ci*

R(X–Y) 2.833 2.841

C2 Ci*

2.814 2.820

C2 Ci*

2.811 2.755

C2* Ci

3.616 3.031

C2 Ci*

3.142 3.128

C2 Ci*

3.146 3.084

R(X–H) 0.972 0.973 0.965 0.976 0.976 0.969 0.978 0.979 0.971 0.972 0.973 0.969 1.018 1.018 1.012b 1.021 1.021 1.019

R(HY) 1.929 1.932

1/2 cannot be measured, all of the O–O couplings are eliminated. Couplings between one nucleus that has I = 1/2 and another with I > 1/2 can be measured, but small differences are not detectable. This is the situation for O–H and O–F couplings. However, progress is being made in this area.36 With ultrahigh resolution NMR, couplings between nuclei with I = 1/2 can be measured to a precision better than 0.1 Hz,37 which means that even small couplings involving H, N, and F could be detected. However, experimental studies of the particular dimers investigated in this study would be difficult both in solution where Published by NRC Research Press

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they would need to form and remain stable, and in the solid state with the very low temperatures required. The most promising candidate for experimental investigation would be a strongly-bound complex with a 15N-labeled hydrazine derivative, which is a solid at room temperature and shows X-ray crystallographic evidence for the existence of hydrogen bonds. Conclusions Corresponding spin–spin coupling constants for hydrogenbonded atoms in isomeric pairs of homo- and hetero-dimers of C2 and Ci symmetry may differ, but the differences are small and would probably be difficult to detect experimentally. For the four complexes with O–HX hydrogen bonds, 1J(O1–H) has a larger absolute value in the C isomer. For 2 the same set of complexes, 1J(O1–O2) has a larger absolute value in the Ci isomer. No distinguishable patterns could be discerned in corresponding spin–spin coupling constants in the C2 and Ci isomers of complexes with N–HN hydrogen bonds.

Acknowledgments This work was carried out with financial support from the Ministerio de Educacioón y Ciencia (Project No. CTQ200913129-C02-02) and Comunidad Autońoma de Madrid (Project MADRISOLAR, ref. S-0505/PPQ/0225). Thanks are given to the Ohio Supercomputer Center for its continued support and to the CTI (Centro Tećnico de Informa´tica; CSIC: Consejo Superior de Investigaciones Cientı´ficas) for an allocation of computer time.

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