The l-alanyl-l-alanine (AA) molecule behaves differently in acidic, neutral, and basic environments. Because of its molecular flexibility and strong interaction with the aqueous environment, its behavior has to be deduced from the NMR spectra indirectly, using statistical methods and comparison with ab initio predictions of geometric and spectral parameters. In this study, chemical shifts and indirect spin-spin coupling constants of the AA cation, anion, and zwitterion were measured and compared to values obtained by density functional computations for various conformers of the dipeptide. The accuracy and sensitivity of the quantum methods to the molecular charge was also tested on the (mono)-alanine molecule. Probable AA conformers could be identified at two-dimensional potential energy surfaces and verified by the comparison of the computed parameters with measured NMR data. The results indicate that, whereas the main-chain peptide conformations of the cationic (AA+) and zwitterionic (AAZW) forms are similar, the anion (AA-) adopts also another, approximately equally populated conformer in the aqueous solution. Additionally, the NH2 group can rotate in the two main chain conformations of the anionic form AA-. According to a vibrational quantum analysis of the two-dimensional energy surfaces, higher-energy conformers might exist for all three charged AA forms but cannot be detected directly by NMR spectroscopy because of their small populations and short lifetimes. In accord with previous studies, the NMR parameters, particularly the indirect nuclear spin-spin coupling constants, often provided an excellent probe of a local conformation. Generalization to peptides and proteins, however, has to take into account the environment, molecular charge, and flexibility of the peptide chain.

With the aid of labeling with stable isotopes ((15)N and (13)C) a complete set of chemical shifts and indirect spin-spin coupling constants was obtained for the zwitterionic form of L-alanyl-L-alanine in aqueous solution. Different sensitivities of the NMR parameters to the molecular geometry were discussed on the basis of comparison with ab initio (DFT) calculated values. An adiabatic two-dimensional vibrational wave function was constructed and used for determination of the main chain torsion angle dispersions and conformational averaging of the NMR shifts and coupling constants. The quantum description of the conformational dynamics based on the density functional theory and a polarizable continuum solvent model agrees reasonably with classical molecular dynamics simulations using explicit solvent. The results consistently evidence the presence of a single form in the aqueous solution with equilibrium main chain torsion angle values (psi = 147 degrees, varphi = -153 degrees), close to that one found previously in an X-ray study. Under normal temperature the torsion angles can vary by about 10 degrees around their equilibrium values, which leads, however, to minor corrections of the NMR parameters only. The main chain heavy atom chemical shifts and spin-spin coupling constants involving the alpha-carbon and hydrogen atoms appear to be most useful for the peptide structural predictions.

Static and frequency-dependent dipole polarizabilities α and first hyperpolarizabilities β are calculated for H2O using self-consistent field (SCF) and multiconfigurational self-consistent- field (MCSCF) linear and quadratic response theory. With an active orbital space where one correlating orbital is included for each occupied valence orbital excellent agreement is obtained with the experimental hyperpolarizability. Basis set dependency has been investigated and a detailed vibrational analysis has been carried out.

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Highly correlated internally contracted multireference configuration interaction wave functions are used to calculate the potential energy and spin-orbit coupling functions for the lowest electronic states of CO2+ dication. Using these functions, the positions and lifetimes of the corresponding vibronic states are evaluated by means of log-phase-amplitude, stabilization, and complex-scaling methods within the framework of a multichannel Schrodinger analysis. For the first time in the literature, the calculated lifetimes are in good agreement with the experiment, thereby proving the reliability of the predicted characteristics and adequacy of the used theory for a theoretical study of other molecular dications.

Pade approximants are able to sum effectively the Rayleigh-Schrodinger perturbation series for the ground state energy of the quartic anharmonic oscillator, as well as the corresponding renormalized perturbation expansion [E.J. Weniger, J. Cizek, and F. Vinette, J. Math. Phys. 34, 571 (1993)]. In the sextic case, Pade approximants are still able to sum these perturbation series, but convergence is so slow that they are computationally useless. In the octic case, Pade approximants are not powerful enough and fail. On the other hand, the inclusion of only a few additional data from the strong coupling domain [E.J. Weniger, Ann. Phys. (N.Y.) (to be published)] greatly enhances the power of summation methods. The summation techniques that we consider are two-point Pade approximants and effective characteristic polynomials. It is shown that these summation methods give good results for the quartic and sextic anharmonic oscillators, and even in the case of the octic anharmonic oscillator, which represents an extremely challenging summation problem, two-point Pade approximants give relatively good results. PACS number(s): 02.70.c, 03.65w, 02.30.Lt I. THE SUMMATION OF DIVERGENT PERTURBATION EXPANSIONS Perturbation theory is one of the few principal methods of approximating solutions to eigenvalue problems in theoretical physics [1-4]. Accordingly, there is an extensive literature. Mathematical aspects of perturbation theory are treated in monographs by Friedrichs [5], Kato

The dependence of N1/9 and C1' chemical shielding (CS) tensors on the glycosidic bond orientation (chi) and sugar pucker (P) in the DNA nucleosides 2'-deoxyadenosine, 2'-deoxyguanosine, 2'-deoxycytidine, and 2'-deoxythymidine was studied using the calculation methods of quantum chemistry. The results indicate that these CS-tensors exhibit a significant degree of conformational dependence on chi and P structural parameters. The presented data test underlying assumptions of currently established methods for interpretation of cross-correlated relaxation rates between the N1/9 chemical shielding tensor and C1'-H1' dipole-dipole (Ravindranathan et al. J. Biomol. NMR 2003, 27, 365-75. Duchardt et al. J. Am. Chem. Soc. 2004, 126, 1962-70) and highlight possible limitations of these methods when applied to DNA.

Interaction-induced dipoles of hydrogen molecules colliding with helium atoms: A new ab initio dipole surface for high-temperature applications JCP: BioChem. Phys. 6, 01B616 (2012) Interaction-induced dipoles of hydrogen molecules colliding with helium atoms: A new ab initio dipole surface for high-temperature applications J. Chem. Phys. 136, 044320 (2012) A theoretical study of the CX2N radicals (X = F, Cl, Br): The effect of halogen substitution on structure, isomerization, and energetics J. Chem. Phys. 136, 044316 (2012) Molecular properties via a subsystem density functional theory formulation: A common framework for electronic embedding J. Chem. Phys. 136, 044104 (2012) Statistical thermodynamics of 1-butanol, 2-methyl-1-propanol, and butanal J. Chem. Phys. 136, 034306 (2012) Additional information on J. Chem. Phys. Attention is focused on the Na ϩ •N 2 complex as part of a study of Na ϩ -containing complexes, which have been implicated in the formation of sporadic sodium layers in the upper atmosphere. The equilibrium structure is found to be linear, in agreement with previous studies. A potential energy hypersurface is calculated at the CCSD͑T͒/aug-cc-pVTZ level of theory, where the N 2 moiety is held fixed, but a wide range of Jacobi bond lengths and bond angles are sampled. This hypersurface is fitted to an analytic form and from this anharmonic vibrational separations are calculated, and compared to harmonic values. Rovibrational energy levels are also calculated from the fitted hypersurface. The best estimate of the interaction energy, ⌬E e is 2770 cm .