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Ab initio MP2/aug'-cc-pVTZ calculations have been performed to determine the structures and binding energies of 22 open and 3 cyclic complexes formed from the sp2 [H2C=PH and HP=PH (cis and trans)] and sp3 [PH2(CH3) and PH3] hybridized phosphorus bases and their corresponding protonated ions. EOM-CCSD calculations have been carried out to obtain 31P-31P and 31P-1H coupling constants across P-H+-P hydrogen bonds. Two equilibrium structures with essentially linear hydrogen bonds have been found along the protontransfer coordinate, except for complexes with P(CH3)H3 + as the proton donor to the sp2 bases. Although the isomer having the conjugate acid of the stronger base as the proton donor lies lower on the potential energy surface, it has a smaller binding energy relative to the corresponding isolated monomers than the isomer with the conjugate acid of the weaker base as the donor. The hydrogen bond of the latter has increased proton-shared character. All of the complexes are stabilized by traditional hydrogen bonds, as indicated by positive values of the reduced coupling constants 2hKP-P and 1KP-H, and negative values of 1hK H-P. 2hJP-P correlates with the P-P distance, a correlation determined primarily by the nature of the proton donor. For open complexes, 1JP-H always increases relative to the isolated monomer, while 1hJH-P is relatively small and negative. 2hJP-P values are quite large in open complexes, but are much smaller in cyclic complexes in which the P-H+-P hydrogen bonds are nonlinear. Thus, experimental measurements of 2hJP-P should be able to differentiate between open and cyclic complexes. © 2007 American Chemical Society.

Ab initio equation-of-motion coupled-cluster singles and doubles method calculations have been performed on iminoboranes RBNH, HBNR, and RBNR, for R = H, CH3, NH2, OH, and F, to evaluate substituent effects on one- and two-bond 15N-11B, 11B-1H, and 15N-1H spin-spin coupling constants. For comparison purposes, 13C-13C coupling constants were evaluated for corresponding isoelectronic molecules RCCH and RCCR. The absolute values of 1J(N-B) and 1J(C-C) increase in the series HBNR and RCCH as the σ-electron-withdrawing ability of R increases. In contrast, NH 2 substitution at B leads to a decrease in the absolute value of 1J(N-B), but OH and F substitution increase 1J(N-B). Disubstitution has dramatically different effects on coupling constants for RCCR and RBNR. In the former, 1J(C-C) more than doubles relative to the corresponding RCCH and HCCH; in the latter, disubstitution of OH and F decreases 1J(N-B) relative to the corresponding monosubstituted derivatives, while NH2 substitution increases 1J(N-B). Changes in one- and two-bond B-H and N-H coupling constants upon substitution are similar to changes observed for 1J(N-B) in the corresponding monosubstituted derivatives RBNH and HBNR. © 2007 American Chemical Society.

EOM-CCSD 11B-7Li coupling constants and B chemical shifts have been computed for Li-diazaborole and its complexes with one H 2O or FLi molecule. B-Li coupling constants for a model compound H2BLi and its complexes with up to 4 H2O or FLi molecules have also been obtained in an attempt to resolve discrepancies between the computed values of these properties for isolated Li-diazaborole and experimentally determined values for boryllithium in a THF solution. The presence of solvent molecules increases the ion-pair character of the B-Li bond, with the result that 1J/(B-Li) decreases systematically as the basicity and the number of solvent molecules increases. In the presence of even a single solvent molecule, the boron chemical shift for Li-diazaborole increases, and approaches the experimental value. The computed results emphasize the role of the solvent in determining these NMR properties. Copyright © 2007 John Wiley & Sons, Ltd.

A systematic ab initio EOM-CCSD study of 15N-15N and 15N-1H spin-spin coupling constants has been carried out for a series of complexes formed from 11 nitrogen bases with experimentally measured proton affinities. When these complexes are arranged in order of increasing proton affinity of the proton-acceptor base and, for each proton acceptor, increasing order of proton affinity of the protonated N-H donor, trends in distances and signs of coupling constants are evident that are indicative of the nature of the hydrogen bond. All two-bond spin-spin coupling constants (2hJ(N-N)) are positive and decrease as the N-N distance increases. All one-bond N-H coupling constants (1J(N-H)) are negative (1K(N-H) are positive). 1J(N-H) is related to the N-H distance and the hybridization of the donor N atom. One-bond H...N coupling constants (1hJ(H-N)) are positive (1hK(H-N) are negative) for traditional hydrogen bonds, but 1hJ(H-N) becomes negative when the hydrogen bond acquires sufficient proton-shared character. The N-N and H...N distances at which 1hJ(H-N) changes sign are approximately 2.71 and 1.62 A, respectively. Predictions are made of the values of 2hJ(N-N) and 1J(N-H), and the signs of 1hJ(H-N), for those complexes that are too large for EOM-CCSD calculations.

Karplus-type equations are derived for the variation of one-bond X-Y coupling constants 1J(X-Y) as a function of dihedral angle for molecules HmX-YHn, for X, Y, = 15N, 17O, 31P, and 33S. Coupling constants were obtained from ab initio EOM-CCSD calculations, with all terms evaluated. The relative orientation of lone pairs appears to be a primary factor determining the dependence of 1J(X-Y) on the dihedral angle. © 2006 American Chemical Society.

Seventy-three unique spin-spin coupling constants have been analyzed for the ten species in the two series X(CH3)nH4-n, where the central atom X is 13C or 15N+. Thirty-seven experimental values have been obtained from the literature, and several new coupling constants have been measured for the methyl-substituted ammonium ions. Both DFT with the B3LYP functional and ab initio EOM-CCSD calculations have been carried out on these same systems. Coupling constants computed by these two methods are in agreement with experimental values. Some problems related to coupling constants for the cationic ammonium systems have been resolved when these were recomputed at EOM-CCSD for complexes in which NH4 + is hydrogen-bonded to H2O molecules. Copyright © 2006 John Wiley & Sons, Ltd.

Ab initio EOM-CCSD calculations have been performed on 3:1 FH:NH 3 complexes at their own optimized MP2/6-31+G(d,p) geometries and at the optimized geometries in the hydrogen-bonding regions of corresponding 3:1 FH:collidine complexes. The isolated gas-phase equilibrium 3:1 FH:NH3 complex has an open structure with a proton-shared Fa-H a-N hydrogen bond, while the isolated equilibrium 3:1 FH:collidine complex has a perpendicular structure with an Fa-Ha-N hydrogen bond that is on the ion-pair side of proton-shared. The Fa-N coupling constant (2hJFa-N) for the equilibrium 3:1 FH:NH3 complex is large and negative, consistent with a proton-shared Fa-Ha-N hydrogen bond; 2hJFb-Fa, is positive, reflecting a short Fb-Fa distance and partial proton transfer from Fb to Fa across the F b-Hb-Fa hydrogen bond. In contrast, 2hJFa-N has a smaller absolute value and 2hJFb-Fa is greater for the 3:1 FH:NH3 complex at the equilibrium 3:1 FH:collidine geometry, consistent with the structural characteristics of the Fa-Ha-N and Fb-H b-Fa hydrogen bonds. Coupling constants computed at proton-transferred 3:1 FH:collidine perpendicular geometries are consistent with experimental coupling constants for the 3:1 FH:collidine complex in solution and indicate that the role of the solvent is to promote further proton transfer from Fa to N across the Fa-Ha-N hydrogen bond, and from Fb to Fa across the two equivalent F b-Hb-Fa hydrogen bonds. The best correlations between experimental and computed coupling constants are found for complexes with perpendicular proton-transferred structures, one having the optimized geometry of a 3:1 FH:collidine complex at an Fa-Ha distance of 1.80 Å, and the other at the optimized 3:1 FH:collidine geometry with distances derived from the experimental coupling constants. These calculations provide support for the proposed perpendicular structure of the 3:1 FH:collidine complex as the structure which exists in solution. © 2006 American Chemical Society.

The computed MP2/6-31+G(d,p) structure of the HBr:pyridine complex and the computed harmonic and experimental spectra of this complex agree that HBr:pyridine is stabilized by a proton-shared hydrogen bond. However, differences are evident between the computed and experimental spectral patterns with respect to the frequencies and intensities of strong bands. Solution of a one-dimensional nuclear vibrational equation along the normal coordinate mode allows for an estimation of an anharmonicity of the proton-stretching motion. When an anharmonic stretching force constant is used to recompute the spectrum, excellent agreement is found between theory and experiment.

A systematic ab initio study has been carried out to determine the MP2/ 6-31 +G(d,p) structures and EOM-CCSD coupling constants across N-H-F-H-N hydrogen bonds for a series of complexes F(H3NH)2 +, F(HNNH2)2 +, F(H 2CNH2)2 +, F(HCNH)2 +, and F(FCNH)2 +. These complexes have hydrogen bonds with two equivalent N-H donors to F-. As the basicity of the nitrogen donor decreases, the N-H distance increases and the N-H-F-H-N arrangement changes from linear to bent. As these changes occur and the hydrogen bonds between the ion pairs acquire increased proton-shared character, 2hJF-N increases in absolute value and 1hJ H-F changes sign. F(H3NH)2 + complexes were also optimized as a function of the N-H distance. As this distance increases and the N-H⋯F hydrogen bonds change from ion-pair to proton-shared to traditional F-H⋯N hydrogen bonds, 2hJ F-N initially increases and then decreases in absolute value, 1JN-H decreases in absolute value, and 1hJ H-F changes sign. The signs and magnitudes of these coupling constants computed for F(H3NH)2 + at short N-H distances are in agreement with the experimental signs and magnitudes determined for the F(collidineH)2 + complex in solution. However, even when the N-H and F-H distances are taken from the optimized structure of F(collidineH)2 +, 2hJ F-N and 1hJH-F are still too large relative to experiment. When the distances extracted from the experimental NMR data are used, there is excellent agreement between computed and experimental coupling constants. This suggests that the N-H-F hydrogen bonds in the isolated gas-phase F(collidineH)2 + complex have too much proton-shared character relative to those that exist in solution. © 2005 American Chemical Society.

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 (1JX-H and 1hJ H-Y) and two-bond (2hJx-y) spin-spin coupling constants in AH:XH:YH3 complexes, where A and X are 19F and 35C1 and Y is either 15N or 31P. 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:YH3 < ClH:XH:YH 3 < FH:XH:YH3. 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 (1KX-H and 2hK H-Y) are positive, consistent with previous studies of complexes in which X and Y are second-period elements in hydrogen-bonded dimers. 1hKH-Y is, as expected, negative in these complexes which have traditional hydrogen bonds, except for C1H:FH:NH3 and FH:FH:NH3. In these two complexes, the F-H-N hydrogen bond has sufficient proton-shared character to induce a change of sign in 1hKH-Y. The effects of trimer formation on spin-spin coupling constants are markedly greater in complexes in which NH3 rather than PH3 is the proton acceptor. © 2005 American Chemical Society.

Investigations of spin–spin coupling constants (JXY2h) across XHY hydrogen bonds constitute a relatively new and exciting area of theoretical and experimental research. Despite the tremendous progress that has been made in a short period of time, there remain many fundamental questions that need to be addressed. Among these are the following: (1) Is it possible to predict the signs of two-bond spin–spin coupling constants (JXY2h) across XHY hydrogen bonds? (2) What determines the sign of JXY2h? (3) Does the measurement of a two-bond coupling constant prove that the hydrogen bond is covalent? (4) What role does the proton play in coupling across hydrogen bonds? Systematic studies of two-bond coupling constants for series of complexes stabilized by CHN, NHN, OHN, FHN, CHO, NHO, OHO, and FHO hydrogen bonds have been carried out using the ab initio equation-of-motion coupled cluster singles and doubles (EOM–CCSD) method. The results of these studies are used to provide some answers to these questions.

Ab initio calculations at the equation-of-motion coupled cluster (EOM-CCSD) level of theory have been carried out to investigate one-bond (13)C-(1)H, (15)N-(1)H, (17)O-(1)H, and (19)F-(1)H coupling constants in a systematic study of monomers and hydrogen-bonded complexes. Computed coupling constants ((1)J(X-H)) for monomers are in good agreement with available experimental data. All reduced Fermi-contact terms and reduced coupling constants ((1)K(X-H)) for monomers and complexes are positive. Plots of (1)K(X-H) versus the X-H distance for the 16 monomers and the 64 complexes in which these monomers are proton donors exhibit significant scatter. However, a linear relationship has been demonstrated for the first time between coupling constants and X-H distances for different X atoms by plotting the ratios of the coupling constants for complexes and corresponding monomers versus the ratios of distances for complexes and corresponding monomers times the square of the Pauling electronegativity. Since the ratio removes the dependence of coupling constants on the magnetogyric ratios of X, this relationship holds for both (1)K(X-H) and (1)J(X-H). The decrease in reduced coupling constants ((1)K(X-H)) as the X-H distance increases is due primarily to the increased proton-shared character of the hydrogen bond.

Ab initio calculations using the equation-of-motion coupled cluster method have been carried out to investigate 19F-19F spin-spin coupling constants for a pair of HF molecules. The overall features of the J(F-F) coupling surface with respect to the F-F distance and the orientation of the pair of HF molecules reflect those of the Fermi-contact (FC) surface, although the FC term may not be a good quantitative estimate of J(F-F). The hydrogen-bonded HF dimer exhibits unusual behavior compared to other hydrogen-bonded complexes, since both the FC term and 2hJ(F-F) exhibit variations in sign and magnitude as the F-F distance changes and the linearity of the hydrogen bond is destroyed. The FC term for F-F coupling is relative small and negative for the equilibrium dimer. At the dimer F-F distance, the maximum negative value for the FC term is found for the linear arrangement F-H...H-F, while the maximum positive value is found for the linear H-F...F-H arrangement, despite the fact that neither of these structures is bound. Changes in the sign and magnitude of the FC term are analyzed using the nuclear magnetic resonance triplet wave function model, which relates the orientation of magnetic nuclei to the phases of the wave functions for excited triplet states that couple to the ground state. The FC term for a particular orientation is a result of competing positive and negative contributions from different triplet states, the sign of each contribution being determined by the alignment of the nuclear magnetic moments in that state. Factors are identified which must play a role in determining which types of wave functions dominate.

The reduced two-bond Fermi-contact terms and the reduced spin-spin coupling constants (2h)K(X-Y) across X-H-Y hydrogen bonds for complexes stabilized by C-H-N, N-H-N, O-H-N, F-H-N, C-H-O, O-H-O, F-H-O and C-H-F hydrogen bonds are positive. The NMR Triplet Wavefunction Model (NMRTWM) indicates that the signs of the reduced FC terms and (2h)K(X-Y) are determined by excited triplet states that have an odd number of nodes intersecting the X-Y axis between X and Y, thereby leading to an antiparallel alignment of the nuclear magnetic moments of atoms X and Y.

Author Institution: Youngatown State University, Youngatown, Ohio 44555; University of Florida, Gainesville, Florida 32611

Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) calculations have been performed to evaluate three-bond (15)N-(31)P coupling constants ((3h)J(N[bond]P)) across N[bond]H....O[bond]P hydrogen bonds in model cationic and anionic complexes including NH(4)(+):OPH, NH(4)(+):OPH(3), NH(3):(-)O(2)PH(2), NFH(2):(-)O(2)PH(2), and NF(2)H:(-)O(2)PH(2). Three-bond coupling constants can be appreciable when the phosphorus is P(V), but are negligible with P(III). (3h)J(N[bond]P) values in complexes with cyclic or open structures are less than 1 Hz, a consequence of the nonlinear arrangement of N, H, O, and P atoms. For complexes with these structures, (3h)J(N[bond]P) may not be experimentally measurable. In contrast, complexes in which the N, H, O, and P atoms are collinear or nearly collinear have larger values of (3h)J(N[bond]P), even though the N[bond]P distances are longer than N[bond]P distances in cyclic and open structures. In linear complexes, (3h)J(N[bond]P) is dominated by the Fermi-contact term, which is distance dependent. Therefore, N[bond]P (and hydrogen-bonding N[bond]O) distances in these complexes can be determined from experimentally measured (15)N-(31)P coupling constants.

A systematic ab initio investigation has been carried out to determine the structures, binding energies, and spin-spin coupling constants of ternary complexes X:CNH:Z and corresponding binary complexes X:CNH and CNH:Z, for X, Z = CNH, FH, ClH, FCl, and HLi. The enhanced binding energies of ternary complexes X:CNH:Z for fixed X as a function of Z decrease in the same order as the binding energies of the binary complexes CNH:Z. In contrast, the enhanced binding energies of the ternary complexes for fixed Z as a function of X do not decrease in the same order as the binding energies of the binary complexes X:CNH, a consequence of the increased stabilities of ternary complexes FCl:CNH:Z due to very strong chlorine-shared halogen bonds. For complexes in which the X⋯CNH interaction is a D-H⋯C hydrogen bond for D-H the proton-donor group (N-H, F-H, or Cl-H), spin-spin coupling constants 1J(D-H) and 2hJ(D-C) in ternary complexes X:CNH:Z decrease in absolute value as the binding energies of binary complexes CNH:Z and the enhanced binding energies of the ternary complexes for fixed X as a function of Z also decrease. However, 2XJ(F-C) increases as the enhanced binding energies of the ternary complexes FCl:CNH:Z decrease, a consequence of the nature of the chlorine-shared halogen bond. The one-bond coupling constants 1J(N-H) for the CNH⋯Z interaction in ternary complexes vary significantly, depending on the nature of the X⋯CNH interaction. The largest values of 1J(N-H) are found for ternary complexes with FCl as X. Two-bond coupling constants 2hJ(N-A) for A the proton-acceptor atom of Z, and 2dJ(N-H) decrease in absolute value in the order of decreasing enhancement energies of ternary complexes X:CNH:Z for fixed Z as a function of X. © the Owner Societies 2011.

An ab initio study of an isomer of diborane(4) [B2H4] has been carried out at MP2/aug-cc-pVTZ to investigate the ground-state properties of this unusual molecule, a derivative of which has been described in the recent literature. The geometric, electronic and orbital characteristics of B2H4(4) have been analyzed using AIM, NBO, and ELF methodologies. A region with a high concentration of electron density is located near and along the B-B bond, on the opposite side of this bond relative to the bridging H atoms. This site serves as an electron-donor site to electrophiles, resulting in hydrogen-bonded complexes of B2H4 with proton donors HF, HNC, HCl, HCN, and HCCH, and a van der Waals complex with H 2. These complexes have C2v symmetry and stabilization energies that vary from 2 to 27 kJ mol-1. The SAPT2 energy decomposition analysis shows that the relative importance of the various terms that contribute to the interaction energy depends on the strength of the interaction. © the Owner Societies 2011.

A review is presented of experimental studies of infrared and Raman spectra of thymine isolated in an argon matrix at about 12 K. The experimental techniques for study of the vibrational spectra of matrix-isolated pyrimidines are described, and the reasons for such studies are illustrated by comparison of the infrared and Raman spectra of matrix-isolated thymine with spectra in the solid phase. Comparison of the infrared spectrum of p-quinone isolated in an argon matrix with that for the molecule in the gas phase establishes that the spectra of matrix-isolated molecules are expected to be very similar to those for the gas phase, except for the lack of rotational structure in the matrix spectra. On the other hand, hydrogen bonding in the solid phase has a profound effect on the infrared spectrum, with much smaller effect on the Raman spectrum. Ab initio quantum-mechanical calculations at the HF/6–3 lG(d) level made for the isolated molecules are compared with the experimental spectra and their application to the interpretation of the latter is illustrated.

EOM-CCSD calculations have been performed to evaluate the 31P−31P coupling constant (4hJP-P) across an N−H+−N hydrogen bond in a model system. Computed 4hJP-P values were obtained as a function of distance and are in agreement with an experimentally measured value of the 31P−31P coupling constant across an N−H+−N hydrogen bond.

Ab initio SCF calculations with a minimal STO−3G basis set have been performed to determine the equilibrium structures and energies of dimers having formamide as the proton donor molecule and either water or formaldehyde as proton acceptor molecules. The structures of dimers in which the N−H proton ’’s−trans’’ to the carbonyl group is hydrogen bonded (t dimers) are consistent with structures anticipated from the general hybridization model. In these dimers, there is essentially free rotation of the proton acceptor molecule about the intermolecular line. When hydrogen bond formation involves the ’’s−cis’’ proton (c dimers), a single equilibrium formamide−water and formamide−formadehyde dimer exists, the structure of which is strongly influenced by the secondary factors of dipole alignment and long−range interaction. These factors are also responsible for the increased stability of c dimers relative to t dimers. A set of 1:2 formamide:water trimers has been constructed from the equilibrium formamide−water d...

Ab initioSCF and SCF-CI calculations with the STO-3G basis set have been performed to investigate the structures and energies of water–cytosine complexes and the intermolecular water–cytosine surface in the cytosine molecular plane. Although there are six nominal hydrogen-bonding sites in this plane, only three dimers are distinguishable in the ground state. The most stable has an energy of −10.7 kcal/mol, and is found in the N1H and O2 region. An asymmetric cyclic structure in which the water molecule bridges adjacent N1H and O2 sites is the preferred form of this dimer. The dimer in the region between O2 and N4H′ of the amino group is slightly less stable at −10.4 kcal/mol, and also has an asymmetric cyclic structure as the preferred structure, with the water molecule bridging amino N4H′ and N3 hydrogen-bonding sites. The third dimer has the amino group as the proton donor to water through the hydrogen cis to C5, and a stabilization energy of −7.0 kcal/mol. The water-cytosine surface is characterized by deeper minima and higher barriers than the water-thymine surface and by a decreased mobility of the water molecule between adjacent hydrogen-bonding sites. Absorption of energy by the C2O group leads to the first n → π* excited state in which interactions of water with O2 are broken. The water-cytosine dimers remain bound in this state, but may change structurally. In the second n → π* state interactions between water and N3 are no longer stabilizing. As a result, the dimer in the O2 and N4H′ region collapses to either a dimer with water the proton donor to O2, or one with N4H′ the proton donor to water. The other two dimers remain bound. All excited dimers are destabilized on vertical excitation relative to the ground state.