Equation-of-motion Coupled-cluster Singles And Doubles Research Articles

Oscillator Strengths in ONIOM Excited State Calculations.

We compute oscillator strengths with the ONIOM (Our own N-layer Integrated molecular Orbital molecular Mechanics) hybrid method between ground and valence excited states and compare the results with the high level of theory equation of motion coupled cluster singles and doubles (EOM-CCSD). This work follows our previous studies in which we validated the ability of ONIOM to compute accurate transition energies compared to EOM-CCSD. We test various levels of theory and molecular systems, as well as the effect of the link atom bond length. Our results show that oscillator strengths can be accurately computed with ONIOM, provided that a sensible choice of the partitioning and of the low level method is made. Being able to calculate both the transition energy and the oscillator strength, ONIOM represents a promising approach to completely characterize valence excited states of molecules that are too large to be studied with a conventional high-accuracy method.

Link atom bond length effect in ONIOM excited state calculations

We investigate how the choice of the link atom bond length affects an electronic transition energy calculation with the so-called our own N-layer integrated molecular orbital molecular mechanics (ONIOM) hybrid method. This follows our previous paper [M. Caricato et al., J. Chem. Phys. 131, 134105 (2009)], where we showed that ONIOM is able to accurately approximate electronic transition energies computed at a high level of theory such as the equation of motion coupled cluster singles and doubles (EOM-CCSD) method. In this study we show that the same guidelines used in ONIOM ground state calculations can also be followed in excited state calculations, and that the link atom bond length has little effect on the ONIOM energy when a sensible model system is chosen. We also suggest further guidelines for excited state calculations which can help in checking the effectiveness of the definition of the model system and controlling the noise in the calculation.

Electronic excitation energies in solution at equation of motion CCSD level within a state specific polarizable continuum model approach

We present a study of excitation energies in solution at the equation of motion coupled cluster singles and doubles (EOM-CCSD) level of theory. The solvent effect is introduced with a state specific polarizable continuum model (PCM), where the solute-solvent interaction is specific for the state of interest. Three definitions of the excited state one-particle density matrix (1PDM) are tested in order to gain information for the development of an integrated EOM-CCSD/PCM method. The calculations show the accuracy of this approach for the computation of such property in solution. Solvent shifts between nonpolar and polar solvents are in good agreement with experiment for the test cases. The completely unrelaxed 1PDM is shown to be a balanced choice between computational effort and accuracy for vertical excitation energies, whereas the response of the ground state CCSD amplitudes and of the molecular orbitals is important for other properties, as for instance the dipole moment.

Using the ONIOM hybrid method to apply equation of motion CCSD to larger systems: Benchmarking and comparison with time-dependent density functional theory, configuration interaction singles, and time-dependent Hartree–Fock

Equation of motion coupled-cluster singles and doubles (EOM-CCSD) is one of the most accurate computational methods for the description of one-electron vertical transitions. However, its O(N(6)) scaling, where N is the number of basis functions, often makes the study of molecules larger than 10-15 heavy atoms prohibitive. In this work we investigate how accurately less expensive methods can approximate the EOM-CCSD results. We focus on our own N-layer integrated molecular orbital molecular mechanics (ONIOM) hybrid scheme, where the system is partitioned into regions which are treated with different levels of theory. For our set of benchmark calculations, the comparison of conventional configuration interaction singles (CIS), time-dependent Hartree-Fock (TDHF), and time-dependent density functional theory (TDDFT) methods and ONIOM (with different low level methods) showed that the best accuracy-computational time combination is obtained with ONIOM(EOM:TDDFT), which has a rms of the error with respect to the conventional EOM-CCSD of 0.06 eV, compared with 0.47 eV of the conventional TDDFT.

Improving upon the accuracy for doubly excited states within the coupled cluster singles and doubles theory

An alternative strategy of computations for double character excited states has been examined. The basic idea is to employ the reference function specific to the excited state of interest, as opposed to the traditionally used reference function, usually corresponding to the ground state, specific to the entire spectrum of a molecule. The procedure is used within the framework of the coupled cluster singles and doubles (CCSD) method. The conventional spin-conserving CC approach as well as its spin-flip (SF) extension has been analyzed. For the latter, two variants are considered, changing the S(z) value of the reference function by one [equation-of-motion (EOM)-SF] and two (EOM-2SF). The accuracy of the methods is benchmarked for the C(2) and C(4) molecules and referred to the full configuration interaction (FCI) or CC singles, doubles, and triples results. The vertical and adiabatic excitation energies, equilibrium geometries, and harmonic frequencies are studied. A significant improvement is demonstrated for the excitation energies of doubly excited states. Comparing these values with the FCI method, the errors of the conventional EOM CCSD method of about 1.7-2.2 eV are reduced to about 0.0-0.4 eV for the SF method. An improvement is also shown for the equilibrium geometries and harmonic frequencies.

A Systematic Comparison of Second-Order Polarization Propagator Approximation and Equation-of-Motion Coupled Cluster Singles and Doubles C−C, C−N, N−N, C−H, and N−H Spin−Spin Coupling Constants

Ab initio one-, two-, and three-bond C-C, C-N, and N-N spin-spin coupling constants, and one-bond C-H and N-H coupling constants have been computed using two different theoretical methods, SOPPA/(qzp,qz2p) and EOM-CCSD/(qzp,qz2p). Both EOM-CCSD (equation-of-motion coupled cluster singles and doubles) and SOPPA (second-order polarization propagator approximation) coupling constants correlate linearly with experimental data. In the great majority of cases, the computed EOM-CCSD C-C, C-N, N-N, and N-H coupling constants are in better agreement with experimental data than SOPPA values, although both levels of theory provide reasonable estimates of these couplings. EOM-CCSD consistently underestimates one-bond C-H coupling constants by about 10 Hz, and SOPPA values of (1)J(C-H) are in better agreement with experimental data. The performance of SOPPA supports its use in future studies of coupling constants involving C, N, and H in larger chemical and biological systems.

Parallel implementation of the equation-of-motion coupled-cluster singles and doubles method and application for radical adducts of cytosine

The equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) method has been implemented into the massively parallel ACES III program using two alternative strategies: (1) storing the entire EOM Hamiltonian matrix prior to diagonalization and (2) recomputing the four-virtual part of the matrix from integrals in a direct mode. The second is found to be far more efficient. EOM-CC shows virtually ideal scaling from 32 to 256 processors. With basis sets as large as 552 functions, the program was applied to determine vertical excitation energies for five cytosine radical adducts of -OH and -H at three sites C5, C6, and N3. These radicals are considered to play an important role in radiation induced DNA damage. The excitation energy spectrum shows two distinct patterns for the lowest transitions distinguishing the C6-OH, C6-H, and N3-H adducts from the C5-OH and C5-H. The results indicate that the two lowest transitions of the C6-OH isomer should contribute to the experimentally observed absorption maximum at 2.88 eV, while the third and fourth transitions of C6-OH and the two lowest transitions of C5-OH contribute to the 3.65 eV absorption maximum. We also report the CCSD with noniterative triples correction [CCSD(T)] relative energies of the C5-OH and C6-OH adducts using 1000 processors.

Systematic Comparison of Second-Order Polarization Propagator Approximation (SOPPA) and Equation-of-Motion Coupled Cluster Singles and Doubles (EOM-CCSD) Spin-Spin Coupling Constants for Molecules with C, N, and O Double and Triple Bonds and Selected F-Substituted Derivatives.

Ab initio EOM-CCSD and SOPPA calculations with the Ahlrichs (qzp,qz2p) basis set have been carried out to evaluate one-, two-, and three-bond spin-spin coupling constants for molecules HmXYHn and HmXYHn for X, Y = (13)C, (15)N, and (17)O, and selected (19)F-substituted derivatives. In the great majority of cases, EOM-CCSD one-bond C-C, C-N, C-O, C-F, N-N, N-O, and N-F coupling constants and three-bond F-F coupling constants are smaller in absolute value than the corresponding SOPPA coupling constants, with the EOM-CCSD values in better agreement with experimental data. SOPPA tends to significantly overestimate the absolute values of large one- and three-bond couplings involving fluorine. The majority of two-bond SOPPA coupling constants are in better agreement with experiment than EOM-CCSD, although differences between EOM-CCSD and experimental values are not dramatic. A statistical analysis of thirty EOM-CCSD and SOPPA coupling constants versus experimental coupling constants demonstrates that better agreement with experiment is found when EOM-CCSD is the computational method.

Structures, Bonding, and One-Bond B-N and B-H Spin-Spin Coupling Constants for a Series of Neutral and Anionic Five-Membered Rings Containing BN Bonds.

The structures and bonding of a series of five-membered rings with BN bonds CxNyBzH5 (x + y + z = 5) and their most stable deprotonated anions CxNyBzH4(-) as well as anionic rings CxNyBzH5(-) have been investigated at the MP2/6-311++G(d,p) level of theory. The great majority of these rings present BN bond orders close to that found in borazine, suggesting that there is substantial electron delocalization in these rings. This observation is also supported by both NBO and ELF analyses. Ab initio equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) calculations have also been performed to obtain the (15)N-(11)B and (1)H-(11)B spin-spin coupling constants. For neutral systems, the former range from -10 to -35 Hz, thereby bracketing the value of (1)J(B-N) for borazine, which is -29 Hz. (1)J(B-N) spans an even greater range in the anions, from -3 to -36 Hz. The absolute value of (1)J(B-N) decreases upon deprotonation if coupling involves the deprotonated nitrogen or a boron atom bonded to the deprotonated N. (1)J(B-H) always decreases upon nitrogen deprotonation.

Resolving an apparent discrepancy between theory and experiment: spin–spin coupling constants for FCCF

Ab initio equation of motion coupled cluster singles and doubles (EOM-CCSD) and second-order polarization propagator approximation (SOPPA) calculations have been performed to evaluate spin-spin coupling constants for FCCF (difluoroethyne). The computed EOM-CCSD value of (3)J(F-F) obtained at the experimental geometry of this molecule supports the previously reported experimental value of 2.1 Hz, thereby resolving an apparent discrepancy between theory and experiment. This coupling constant exhibits a strong dependence on the C-C and C-F distances, and its small positive value results from a sensitive balance of paramagnetic spin-orbit (PSO) and spin-dipole (SD) terms. The three other unique FCCF coupling constants (1)J(C-C), (1)J(C-F), and (2)J(C-F) have also been reported and compared with experimental data. While (1)J(C-F) is in agreement with experiment, the computed value of (2)J(C-F) is larger than our estimate of the experimental coupling constant.

On the Accuracy of Computed Excited-State Dipole Moments

The dipole moments of furan and pyrrole in many electronically excited singlet states have been determined using coupled cluster theory including large one-electron basis sets. The inclusion of connected triple excitations is shown to uniformly decrease the equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) excitation energies by 0.04-0.24 eV, with an average reduction of 0.08 eV. Using a basis set larger than DZP (++)D (double-zeta plus polarization augmented with atom- and molecule-centered diffuse functions) uniformly increases the computed EOM-CCSD excitation energies by 0.03-0.29 eV, with an average increase of 0.20 eV. The corresponding shifts in excited-state dipole moments are more erratic. Including connected triple excitations changes the computed dipole moments by an rms amount of 0.17 au. More importantly, using a larger basis set shifts the dipole moments by an rms amount of 0.52 au, with an increase or a decrease being equally likely. The CC dipole moments are compared to those from time-dependent density functional theory (TD-DFT) computed by Burcl, Amos, and Handy [ Chem. Phys. Lett. 2002, 355, 8]. For 29 excited states of furan and pyrrole, the predicted TD-DFT dipole moments differ from the CC results by rms amounts of 1.6 au (HCTH functional) and 1.5 au (B97-1 functional). Including the asymptotic correction to TD-DFT developed by Tozer and Handy [ J. Chem. Phys. 1998, 109, 10180; J. Comput. Chem. 1999, 20, 106] reduces the rms differences for both functionals to 1.2 au. If those Rydberg excited states with very large polarizabilities are excluded, the rms differences from the CC results for the remaining 17 excited states become 1.31 au (HCTH) and 0.88 au (B97-1). For asymptotically corrected functionals and this subset of states, the rms differences from the CC results are only 0.54 au (HCTHc) and 0.34 au (B97-1c). Thus, the Tozer-Handy asymptotic correction for TD-DFT significantly improves the predictions of excited-state dipole moments. For excited states without very large polarizabilities, good agreement is achieved between excited-state dipole moments computed by coupled cluster theory and by the asymptotically corrected B97-1c density functional.

A Systematic Comparison of Second-Order Polarization Propagator Approximation (SOPPA) and Equation-of-Motion Coupled Cluster Singles and Doubles (EOM-CCSD) Spin-Spin Coupling Constants for Selected Singly Bonded Molecules, and the Hydrides NH3, H2O, and HF and Their Protonated and Deprotonated Ions and Hydrogen-Bonded Complexes.

Second-order polarization propagator approximation (SOPPA) and equation-of-motion coupled cluster singles and doubles (EOM-CCSD) methods have been employed for the calculation of one-bond spin-spin coupling constants in series of small molecules and ions, and of one- and two-bond coupling constants across X-H···Y hydrogen bonds. For isolated molecules, one-bond SOPPA coupling constants (1)J(X-Y) involving (13)C, (15)N, (17)O, and (19)F have larger absolute values than corresponding EOM-CCSD coupling constants, with the EOM-CCSD values being in significantly better agreement with available experimental data. The difference between SOPPA and EOM-CCSD tends to increase as the number of nonbonding electrons on the coupled atoms increases, and the SOPPA values for O-F coupling are significantly in error. Similarly, the absolute values of SOPPA one-bond coupling constants (1)J(X-H) for the hydrides NH3, H2O, and FH and their protonated and deprotonated ions are greater than EOM-CCSD values, with the largest differences occurring for F-H coupling. One- and two-bond coupling constants (1)J(X-H), (1h)J(H-Y), and (2h)J(X-Y) across X-H···Y hydrogen bonds in neutral, protonated, and deprotonated complexes formed from the hydrides are similar at SOPPA and EOM-CCSD, with the largest differences again found for (1)J(F-H) in complexes with F-H as the proton donor, and (2h)J(F-F) for (FHF)(-). The signs of (1)J(X-H), (1h)J(H-Y), and (2h)J(X-Y) are the same at both levels of theory, as is their variation across the proton-transfer coordinate in F-H···NH3. SOPPA would appear to provide a reliable and more cost-effective alternative approach for computing coupling constants across hydrogen bonds, although couplings involving F may be problematic.

19F– 19F and 19F– 1H spin–spin coupling constants in cyclic FH polymers (FH) n, n=2–6

Spin–spin coupling constants 2h J(F–F), 1 J(F–H), and 1h J(H–F) have been obtained for cyclic complexes (FH) n , with n=2–6, from ab initio equation-of-motion coupled cluster singles and doubles (EOM-CCSD) calculations. Although both the Fermi-contact (FC) term and 2h J(F–F) increase and become positive as the cluster size increases, the FC term is not a good quantitative approximation to 2h J(F–F). The paramagnetic spin–orbit (PSO) and spin–dipole (SD) terms which contribute to 2h J(F–F) appear to be sensitive to the orientation of the hydrogen-bonded pair. However, the large increase in the FC term and 2h J(F–F) as the size of the cluster increases is due primarily to the reorganization of σ electron densities in both ground and excited states, and is another manifestation of cooperativity effects in hydrogen-bonded cyclic polymers. The FC term and 1 J(F–H) always increase upon complex formation, but 1 J(F–H) increases only slightly as the size of the cluster increases due to a concurrent decrease in the PSO term. The changes in 1 J(F–H) as a function of polymer size reflect the polarization of electron density away from H and toward F in the ground state, and the electron reorganization which occurs in the excited states which couple to the ground states through the FC and PSO operators. The FC term is a good approximation to 1h J(H–F), and is always negative, indicating that the hydrogen bonds in the FH clusters are traditional hydrogen bonds.

Second- and third-order triples and quadruples corrections to coupled-cluster singles and doubles in the ground and excited states

Second- and third-order perturbation corrections to equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) incorporating excited configurations in the space of triples [EOM-CCSD(2)T and (3)T] or in the space of triples and quadruples [EOM-CCSD(2)TQ] have been implemented. Their ground-state counterparts--third-order corrections to coupled-cluster singles and doubles (CCSD) in the space of triples [CCSD(3)T] or in the space of triples and quadruples [CCSD(3)TQ]--have also been implemented and assessed. It has been shown that a straightforward application of the Rayleigh-Schrodinger perturbation theory leads to perturbation corrections to total energies of excited states that lack the correct size dependence. Approximations have been introduced to the perturbation corrections to arrive at EOM-CCSD(2)T, (3)T, and (2)TQ that provide size-intensive excitation energies at a noniterative O(n(7)), O(n(8)), and O(n(9)) cost (n is the number of orbitals) and CCSD(3)T and (3)TQ size-extensive total energies at a noniterative O(n(8)) and O(n(10)) cost. All the implementations are parallel executable, applicable to open and closed shells, and take into account spin and real Abelian point-group symmetries. For excited states, they form a systematically more accurate series, CCSD<CCSD(2)T<CCSD(2)TQ<CCSD(3)T<CCSDT, with the second- and third-order corrections capturing typically approximately 80% and 100% of such effects, when those effects are large (>1 eV) and the ground-state wave function has single-determinant character. In other cases, however, the corrections tend to overestimate the triples and quadruples effects, the origin of which is discussed. For ground states, the third-order corrections lead to a rather small improvement over the highly effective second-order corrections [CCSD(2)T and (2)TQ], which is a manifestation of the staircase convergence of perturbation series.

Complexes with N−H+−P Hydrogen Bonds: Structures, Binding Energies, and Spin−Spin Coupling Constants

Ab-initio MP2/aug'-cc-pVTZ calculations have been performed to determine the structures and binding energies of proton-bound complexes stabilized by N-H+-P hydrogen bonds and to investigate the nature of the proton-transfer coordinate in these systems. Double minima are found only when the difference between the protonation energies of the N and P bases is less than about 4 kcal/mol. The isomer in which the protonated nitrogen base is the donor lies lower on the potential surface and also has a greater binding energy relative to the corresponding isolated monomers. Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) calculations have been employed to obtain one- and two-bond spin-spin coupling constants across these hydrogen bonds. Two-bond coupling constants (2h)J(N-P) correlate with N-P distances, irrespective of whether the donor ion is N-H+ or P-H+. One-bond coupling constants (1)J(N-H) and (1h)J(H-P) for complexes stabilized by N-H+...P hydrogen bonds correlate with corresponding distances, but similar correlations are not found for (1)J(P-H) and (1h)J(H-N) for complexes with P-H+...N hydrogen bonds. Negative values of (1h)K(H-N) and (1h)K(H-P) indicate that the hydrogen bonds in these complexes are traditional. Comparisons are made with complexes stabilized by N-H+-N and P-H+-P hydrogen bonds.

A coupled cluster study of the electronic spectroscopy and photochemistry of Cr(CO)6

The transition energies to the low-lying singlet and triplet excited states of Cr(CO)(6) are computed by equation-of-motion coupled cluster singles and doubles (EOM-CCSD) and similarity transformed equation-of-motion coupled cluster singles and doubles (STEOM-CCSD) methods with all-electrons basis sets. Both experimental and optimized geometries are used for the calculations. Calculations with various basis sets, among them one of the largest calculations performed at the EOM-CCSD level, based on atomic natural orbitals with 627 functions, were used to evaluate the basis set influence on computed transition energies. The presence of a shoulder at 3.9 eV in the experimental absorption spectrum, assigned to the (1)A(1g)-->(1)T(2u) transition, which was not reproduced by recent density functional theory (DFT) or multi-state complete active space perturbation theory (MS-CASPT2) is supported by the present STEOM-CCSD calculations with a theoretical value of 3.92 eV. In addition to this weak (1)A(1g)--> a (1)T(2u) absorption, we observe two strong absorptions corresponding to (1)A(1g)--> a (1)T(1u) at 4.37 eV (vs. an experimental value of 4.46 eV) and (1)A(1g)--> b (1)T(1u) at 5.20 eV (vs. an experimental value of 5.53 eV). Both are characterized as metal-to-ligand charge-transfer (MLCT) allowed transitions. The first metal-centered (MC) absorption at 4.37 eV in our best calculation is degenerate with the lowest MLCT absorbing state. The one-dimensional potential energy curves associated to the low-lying singlet MLCT and MC states as a function of the chromium axial carbonyl bond distance q(a) = [Cr-CO(axial)] show that an avoided crossing exists between the a (1)T(1g) (MC) and a (1)T(1u) (MLCT) states near 1.92 A, which is very close to the equilibrium Cr-CO distance. Moreover, the MC state seems to be dissociative for the CO loss. These two important features could explain the ultra-fast dissociation of CO (100 fs) observed in recent low intensity laser probed gas phase experiments.

An ab Initio Study of 15N−11B Spin−Spin Coupling Constants for Borazine and Selected Derivatives

Ab initio equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) calculations have been performed to investigate substituent effects on coupling constants for borazine and selected substituted borazines. For molecules in which F atoms are not bonded to adjacent atoms in the ring, F substitution increases the one-bond (11)B-(15)N coupling constants involving the atom at which substitution occurs but leaves the remaining one-bond B-N coupling constants essentially unchanged. For these molecules, the magnitudes of one-bond B-N coupling constants are only slightly dependent on the number of F atoms present. Fluorine substitution at adjacent B and N atoms in the borazine ring further increases the one-bond B-N coupling constant involving the substituted atoms and has the same effect on the other one-bond coupling constants as observed for corresponding molecules in which substitution occurs at alternate sites. In contrast to the effect of F substitution, substitution of Li at either N or B decreases one-bond B-N coupling constants relative to borazine. The effects of F and Li substitution on one-bond B-N coupling constants for borazine are similar to F and Li substitution effects on (13)C-(13)C coupling constants for benzene.

Ab Initio Study of the Influence of Trimer Formation on One- and Two-Bond Spin−Spin Coupling Constants Across an X−H−Y Hydrogen Bond: AH:XH:YH3 Complexes for A, X = 19F, 35Cl and Y = 15N, 31P

Ab initio equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) calculations have been carried out to investigate the effect of a third polar near-neighbor on one-bond ((1)J(X)(-)(H) and (1h)J(H)(-)(Y)) and two-bond ((2h)J(X)(-)(Y)) spin-spin coupling constants in AH:XH:YH(3) complexes, where A and X are (19)F and (35)Cl and Y is either (15)N or (31)P. The changes in both one- and two-bond spin-spin coupling constants upon trimer formation indicate that the presence of a third molecule promotes proton transfer across the X-H-Y hydrogen bond. The proton-shared character of the X-H-Y hydrogen bond increases in the order XH:YH(3) < ClH:XH:YH(3) < FH:XH:YH(3). This order is also the order of decreasing shielding of the hydrogen-bonded proton and decreasing X-Y distance, and is consistent with the greater hydrogen-bonding ability of HF compared to HCl as the third molecule. For all complexes, the reduced X-H and X-Y spin-spin coupling constants ((1)K(X)(-)(H) and (2h)K(X)(-)(Y)) are positive, consistent with previous studies of complexes in which X and Y are second-period elements in hydrogen-bonded dimers. (1h)K(H)(-)(Y) is, as expected, negative in these complexes which have traditional hydrogen bonds, except for ClH:FH:NH(3) and FH:FH:NH(3). In these two complexes, the F-H-N hydrogen bond has sufficient proton-shared character to induce a change of sign in (1h)K(H)(-)(Y). The effects of trimer formation on spin-spin coupling constants are markedly greater in complexes in which NH(3) rather than PH(3) is the proton acceptor.

Do coupling constants and chemical shifts provide evidence for the existence of resonance-assisted hydrogen bonds?

Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) calculations have been performed to determine two-bond 17O–17O and 15N–15N spin–spin coupling constants (2h J O–O and 2h J N–N) for ten neutral structures which may exhibit intramolecular O–H–O or N–H–N hydrogen bonds. MP2 chemical shifts of hydrogen-bonded protons have also been evaluated. The molecules include malonaldehyde and its diaza counterpart, and the corresponding saturated analogues. The aim of this study is to investigate whether the magnetic properties of the two-bond spin–spin coupling constants and the chemical shifts of the hydrogen-bonded protons provide evidence for the existence of resonance-assisted hydrogen bonds (RAHBs). The two-bond coupling constant for the equilibrium structures is greater for the hydrogen-bonded unsaturated molecule than for the saturated molecule, a result of the shorter O–O distance and stronger hydrogen bond in the former. However, when the O–O or N–N distances are forced to be the same in corresponding saturated and unsaturated structures, the coupling constants are similar. There is a significant increase in coupling constants when the intramolecular hydrogen bond (IMHB) changes from asymmetric to symmetric, due to much shorter O–O or N–N distances. Symmetrization effects also significantly affect the value of the proton chemical shift. For hydrogen-bonded conformers the trends in chemical shifts and coupling constants are similar. A detailed analysis of the NMR properties of oxygen-containing systems leads to the conclusion that neither the coupling constants nor the proton chemical shifts provide any evidence for the existence of RAHBs. Furthermore, the strength of the O–H···O IMHBs, investigated through the use of appropriate homodesmotic reactions, indicates that the enhanced stability of the IMHB in the unsaturated compound is associated with the σ skeleton of the molecule that allows the oxygen atoms to be in closer proximity than in the saturated analogue.

Two-Bond Spin−Spin Coupling across a Hydrogen Bond: X−Y Coupling in the Presence and Absence of the Proton

This paper addresses the question of whether spin−spin coupling between X and Y across an X−H−Y hydrogen bond is proof that the hydrogen bond is covalent. The results of ab initio equation-of-motion coupled cluster singles and doubles (EOM-CCSD) calculations are used to compare coupling constants (2hJX-Y) for hydrogen-bonded complexes and related complexes that have the same X−Y distance but no hydrogen bond. The results show that even in the absence of a hydrogen bond, X−Y coupling can occur. The magnitude of this coupling constant may be greater or less than it is in the hydrogen-bonded complex. Thus, these data suggest that X−Y spin−spin coupling is not a proof of covalency. The role of the hydrogen bond is to allow X and Y to approach each other close enough to form a stable complex, for which an X−Y coupling constant can be measured experimentally. The presence of the proton does influence the magnitude of the coupling constant by altering the s-electron densities on X and Y.