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  • American Institute of Physics (AIP)  (16)
  • 1
    Electronic Resource
    Electronic Resource
    Vibrational frequencies and intensities of H-bonded and Li-bonded complexes. H3N⋅⋅HCl and H3N⋅⋅LiCl (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 3131-3138 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The geometries, energetics, and vibrational spectra are calculated for the two complexes at the SCF and correlated MP2 levels using the 6-31G** basis set, augmented by a second set of d functions on Cl. While correlation represents an important factor in the binding of H3 N⋅⋅HCl, it contributes little to the stronger Li bond. Unlike the HCl stretch νs which decreases substantially in frequency and is greatly intensified in H3 N⋅⋅HCl, the frequency of the LiCl stretch undergoes an increase and little change is noted in its intensity, conforming to prior spectral measurements. The intensities of the intramolecular stretching modes of NH3 are greatly strengthened by formation of a H bond and even more so for a Li bond. These intensity patterns are analyzed via atomic polar tensors which reveal that formation of a H bond dramatically lessens the ability of the electron density to shift along with the proton. A stretch of H–Cl hence leads to a large increase in molecular dipole moment. This "freezing'' of the electron cloud is much smaller in the Li bond and its effect on the νs intensity is counteracted by a much reduced Li atomic charge in the complex. Another distinction between the H and Li bonds relates to the destination of charge transferred from the NH3 subunit which accumulates on Cl in the former case but on Li in the latter.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.454970
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  • 2
    Electronic Resource
    Electronic Resource
    Calculation of barriers to proton transfer using multiconfiguration self-consistent-field methods. I. Effects of localization (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 7507-7518 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The usefulness of multiconfiguration self-consistent-field (MCSCF) calculations in computing correlated proton transfer potentials is investigated for the systems HF2−, H7N2+, H3O2−, and H5O2+. In deciding whether to include particular molecular orbitals, it is important to consider the balance of electron density between the donor and acceptor groups and the interactions that are incorporated in the orbitals. Only orbitals which have the proper symmetry to interact with the transferring hydrogen need be included in the MCSCF active space. Reasonable transfer barriers are obtained when the orbitals are balanced and only interactions relevant to the transfer process are allowed in the MCSCF active space. Equivalent barriers are determined, but the criteria are more easily met, if the canonical molecular orbitals are first subjected to a localization. Only the two localized molecular orbitals that contain the F, N, or O interaction with the transferring hydrogen are needed, which reduces the difficulty of eliminating unproductive interactions. In addition, the localization allows additional virtual orbitals to be included without producing an undesirable correlation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463522
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  • 3
    Electronic Resource
    Electronic Resource
    Calculation of barriers to proton transfer using variations of multiconfiguration self-consistent-field methods. II. Configuration interaction (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 7519-7527 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Various means are tested of including additional electron correlation into multiconfiguration self-consistent-field (MCSCF) methods for computing proton transfer potentials in HF2−, H7N2+, H3O2−, and H5O2+. Configuration interaction allowing single excitations (CIS) and configuration interaction with single + double excitations (CISD) calculations are performed following MCSCF expansion of the wave function using various different MCSCF reference wave functions. The CISD results are excellent, being fairly independent of choice of reference space although it is important that the occupied orbitals be balanced between the donor and acceptor. Localizing the occupied molecular orbitals prior to the MCSCF part of the calculation results in a further improvement since it is possible to use a smaller number of occupied orbitals and thereby allow more virtuals to be included. These results are compared to configuration interaction computations using the canonical orbitals and which are not preceded by MCSCF preparation of the wave function.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463523
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  • 4
    Electronic Resource
    Electronic Resource
    Theoretical study of hydrogen bonding and proton transfer in the ground and lowest excited singlet states of tropolone (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 9755-9765 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Theoretical models of hydrogen bonding and proton transfer in the ground (S0) and lowest excited ππ* singlet (S1) states of tropolone are developed in terms of the localized OH...O fragment model and ab initio three-dimensional potential energy surfaces (PESs). The PESs for proton transfer in the S0 and S1 states are calculated using ab initio SCF and CIS methods, respectively, with a 6–31G basis set which includes polarization functions on the atoms involved in the internal H bond. The Schrödinger equation for nuclear vibrations is solved numerically using adiabatic separation of the variables. The calculated values for the S0 state (geometry, relaxed barrier height, vibrational frequencies, tunnel splittings and H/D isotope effects) agree fairly well with available experimental and theoretical data. The calculated data for the S1 state reproduce the principal experimental trends, established for S1←S0 excitation in tropolone, but are less successful with other features of the dynamics of the excited state, e.g., the comparatively large value of vibrationless level tunnel splitting and its irregular increase with O...O excitation in S1. In order to overcome these discrepancies, a model 2-D PES is constructed by fitting an analytical approximation of the CIS calculation to the experimental vibrationless level tunnel splitting and O...O stretch frequency of tropolone–OH. It is found that the specifics of the proton transfer in the S1 state are determined by a relatively low barrier (only one doublet of the OH stretch lies under the barrier peak). Bending vibrations play a minor role in modulation of the proton transfer barrier, so correct description of tunnel splitting of the proton stretch levels in both electronic states can be obtained in terms of the two-dimensional stretching model, which includes O...O and O–H stretching vibration coordinates only. © 1994 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467941
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  • 5
    Electronic Resource
    Electronic Resource
    Ab initio study of He(1S)+Cl2(X 1Σg,3Πu) potential energy surfaces (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 6800-6809 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The potential energy surface of the ground state He+Cl2(1Σg) is calculated by using the perturbation theory of intermolecular forces and supermolecular Møller–Plesset perturbation theory approach. The potential energy surface of the first excited triplet He+Cl2(3Πu) was evaluated using the supermolecular unrestricted Møller–Plesset perturbation theory approach. In the ground state two stable isomers are found which correspond to the linear He–Cl–Cl structure (a primary minimum, De=45.1 cm−1, Re=4.25 A(ring)) and to the T-shaped structure with He perpendicular to the molecular axis (a secondary minimum, De=40.8 cm−1, Re=3.5 A(ring)). The small difference between these geometries is mainly due to the induction effect which is larger for the linear form. The results obtained for the T-shaped minimum are in good agreement with the excitation spectroscopy experiments which observed only the T-shaped form [Beneventi et al., J. Chem. Phys. 98, 178 (1993)]. In the lowest triplet states correlating with Cl2(3Πu), 3A' and 3A‘, the same two isomers correspond to minima. Now, however, the T-shaped form is lower in energy. The 3A' and 3A‘ states correspond to (De,Re) of (19.9 cm−1, 3.75 A(ring)) and (30.3 cm−1, 3.50 A(ring)), respectively, whereas the linear form is characterized by (19.8 cm−1, 5.0 A(ring)). The binding energy for the T form in the lower 3A‘ state is in good agreement with the experimental value of Beneventi et al.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468308
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  • 6
    Electronic Resource
    Electronic Resource
    Structure, energetics, and vibrational spectrum of H2O–HCl (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 5928-5936 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: H2O–HCl is studied using a number of basis sets including 6-31G** and variants which are augmented by a diffuse sp shell and a second set of d functions on O and Cl. Optimization of the geometry of the complex is carried out including explicitly electron correlation and counterpoise correction of the basis set superposition error (BSSE) at both the SCF and correlated levels. Correlation strengthens and shortens the H bond while BSSE correction leads to an opposite trend; these two effects are of different magnitude and hence cancel one another only partially. ΔH°(298 K) is calculated to be −4.0 kcal/mol, 1/4 of which is due to correlation. Formation of the complex causes the strong intensification and red shift of the H–Cl stretching band normally associated with H bonding, whereas the internal vibrations of H2O are very little affected, except for a doubling of the intensity of the symmetric stretch. With respect to the intermolecular modes, the bends of the proton donor are of higher frequency than those involving the acceptor. While these intermolecular bends are all of moderate intensity, comparable to the intramolecular modes, the H-bond stretch νσ is very weak indeed, consistent with a principle involving subunit dipoles. All calculated vibrational data are in excellent agreement with the spectra measured in solid inert gas matrices.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453516
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  • 7
    Electronic Resource
    Electronic Resource
    Characterization of ground and excited electronic state deprotonation energies of systems containing double bonds using natural bond orbital analysis (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 4675-4691 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Natural bond orbital analysis is applied to the ground and excited states of a set of neutral, cationic, and anionic doubly bonded species HnC=XHn (X=C, N, O) isoelectronic with ethylene. The character of the excitation is correlated with calculated charge shifts and geometry changes upon relaxation. For these planar molecules, depopulation of the π bond or population of the π* antibond causes an out-of-plane twist or pyramidalization upon relaxation correlated to the amount of charge shift. These nonplanar distortions generally lower the energy more than changes in bond lengths and angles. Population of a σXH* antibond by more than ∼0.4 e often leads to dissociation of that proton. The character and symmetry of the transition are used to match excited states in the protonated and deprotonated species so as to extract an excited state deprotonation energy. The vertical deprotonation energy of the π→π* state tends to be higher than that of the ground state due to greater electronic destabilization of the deprotonated species, while Rydberg excited states take less energy to deprotonate. Adiabatic deprotonation energies can be greater or less than that of the ground state depending on whether the protonated or deprotonated species is more stabilized by geometry relaxation. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472309
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  • 8
    Electronic Resource
    Electronic Resource
    Vibrational frequencies and intensities of H-bonded systems. 1:1 and 1:2 complexes of NH3 and PH3 with HF (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 2214-2224 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Frequencies and intensities are calculated by ab initio methods for all vibrational modes of the 1:1 H3X–HF and 1:2 H3X–HF–HF complexes (X=N,P). The HF stretching frequencies are subject to red shifts, roughly proportional to the strength of the H bond, and to manyfold increases in intensity. Although the intramolecular frequency shifts within the proton acceptors are relatively modest, the intensities of the NH3 stretches are magnified by several orders of magnitude as a result of H bonding (in contrast to PH3 which exhibits little sensitivity in this regard). The frequencies and intensities corresponding to bending of the H3X–HF H-bond rise with increasing H-bond strength while the properties of the other intermolecular modes appear somewhat anomalous at first sight. The intensity patterns are analyzed by means of atomic polar tensors which reveal that intensification of the proton donor stretch is chiefly due to increasing charge flux associated with H-bond formation. The different behavior of the N–H and P–H stretching intensities is attributed to the opposite sign of the hydrogen atomic charges in the two molecules. As a general rule, low intensities can be expected for intermolecular modes with the exception of those which involve motions of hydrogens that appreciably alter the magnitude or direction of a subunit's dipole moment.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453148
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  • 9
    Electronic Resource
    Electronic Resource
    Primary and secondary basis set superposition error at the SCF and MP2 levels. H3N--Li+ and H2O--Li+ (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 1194-1204 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The primary basis set superposition error (BSSE) results from the artificial lowering of the energy of each subunit of a pair by the presence of "ghost orbitals'' of its partner. In addition, these ghost orbitals perturb the one-electron properties of the molecule, causing a change in the interaction energy, an effect known as secondary BSSE which is not corrected by the counterpoise procedure. The primary and secondary BSSE are calculated for the interactions of NH3 and H2O with Li+, using a variety of different basis sets. It is found that the 2° BSSE can be quite large, comparable in magnitude to the 1° component at both the SCF and MP2 levels. There is no basis found for the supposition that 2° BSSE improves the calculated interaction energy, nor do the 1° and 2° effects cancel one another in general. While the MP2 BSSE tends to be smaller than the SCF analog, the former can be similar in magnitude to the "true'' MP2 contribution to the interaction; failure to remove the BSSE can hence lead to a qualitatively incorrect interpretation of the effects of electron correlation. Comparison with a system in which basis set superposition is rigorously excluded suggests that subtraction of both the full 1° and 2° BSSE is appropriate and does not overcorrect the potential. Addition of a diffuse sp shell, especially if coupled with orbital exponent reoptimization, leads to a lowering of the 1° and 2° BSSE, which moreover take on opposite sign and cancel one another to some extent.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453299
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  • 10
    Electronic Resource
    Electronic Resource
    Comparison of methods for calculating the properties of intramolecular hydrogen bonds. Excited state proton transfer (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 849-858 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A series of molecules related to malonaldehyde, containing an intramolecular H-bond, are used as the testbed for a variety of levels of ab initio calculation. Of particular interest are the excitation energies of the first set of valence excited states, nπ* and ππ*, both singlet and triplet, as well as the energetics of proton transfer in each state. Taking coupled cluster results as a point of reference, configuration interaction-singles–second-order Møller–Plesset (CIS–MP2) excitation energies are too large, as are CIS to a lesser extent, although these approaches successfully reproduce the order of the various states. The same may be said of complete active space self-consistent-field (CASSCF), which is surprisingly sensitive to the particular choice of orbitals included in the active space. Complete active space–second-order perturbation theory (CASPT2) excitation energies are rather close to coupled cluster singles and doubles (CCSD), as are density functional theory (DFT) values. CASSCF proton transfer barriers are large overestimates; the same is true of CIS to a lesser extent. MP2, CASPT2, and DFT barriers are closer to coupled cluster results, although yielding slight underestimates. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479371
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