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This study deals with the identification of glutamic acid by means of quantum chemical approach. FT-IR, FT-Raman and UV–vis spectra were recorded in the region 4000–400, 4000–50 cm− 1and 200–600 nm, respectively. CAM-B3LYP/6-31G(d,p) and B2PLYP/6-31G(d,p) calculations were performed to obtain the optimised molecular structures, vibrational frequencies and corresponding vibrational assignment, thermodynamic properties and natural bonding orbital (NBO) analysis. The results show that the obtained optimised geometric parameters (bond lengths, bond angles and bond dihedrals) and vibrational frequencies were found to be in good agreement with the experimental results. The calculations of the electronic spectra were compared with the experimental ones. Furthermore, highest occupied molecular orbital and lowest unoccupied molecular orbital analyses and UV–vis spectral analysis were also performed to determine the energy band gaps and transition states. NBO analysis, calculated using density functional theory methods (CAM-B3LYP/6-31G(d,p) and B2PLYP/6-31G(d,p)), was induced to find inter-molecular atoms.13C and1H NMR isotropic chemical shifts were calculated and the assignments made were compared with the ChemDraw Ultra values. [ABSTRACT FROM AUTHOR]

Conformational studies were carried out on acetylcholine and two nerve agents (NAs), soman and sarin, to determine the influence of the low-energy conformers of NAs in inhibiting the hydrolysis of acetylcholine using state-of-the-art quantum chemical techniques. Different enantiomers of sarin and soman were taken into account during the conformational analyses. The aqueous solvation of these molecules was studied at the density functional level using the polarized continuum model with the conductor-like reaction field approach. The results show that, compared with acetylcholine, the NAs have less conformational flexibility in the gas phase. The conformational flexibility of these molecules increases during aqueous solvation, and the transition barrier to different low-energy conformations also decreases. This property, together with the molecular electrostatic potential surfaces of the NAs, was compared with those of acetylcholine to obtain an insight into the cholinesterase inhibition properties of these molecules. [ABSTRACT FROM AUTHOR]

Analytical energy gradients have been implemented in the DALTON program at the level of explicitly correlated second-order Møller-Plesset perturbation theory (MP2-R12). The implementation has been accomplished for standard approximation A using an auxiliary basis which is the same as the orbital basis for the resolution-of-the-identity, with and without frozen core. As a test calculation, the geometries of nine stationary points on the potential energy surface of the water dimer have been optimized and the results are compared with earlier studies. [ABSTRACT FROM AUTHOR]

The high-accuracy W4 computational thermochemistry protocol, and several post-W4 methods, have been applied to the P2 and P4 molecules. Contrary to previous studies, we find the experimental thermochemistry to be fundamentally sound. The reaction enthalpy for P4→2P2 has a very significant contribution from post-CCSD(T) correlation effects. We derive a gas-phase heat of formation for the phosphorus atom of [image omitted] kcal mol-1 and [image omitted] kcal mol-1, in the upper half of the CODATA uncertainty interval. [ABSTRACT FROM AUTHOR]

A hydroiodic acid molecule is dissociated by more than four water molecules. Here, the effect of an excess electron on the hexahydated hydroiodic acid where the dissociated structure [H3O+(H2O)5I-] is much more stable than the undissociated one [(H2O)6HI], is investigated. Upon binding an excess electron (e-), the cluster releases a hydrogen radical and forms the stable hexahydrated iodide [(H2O)6I-] when the initial kinetic energy is above ∼200 K, due to the small transition barrier (∼0.5 eV). The system with the hydrogen radical released, [(H2O)6I- + H·], is much more stable than the systems with an excess electron, [(H3O)+{e-(H2O)5}I-] or [e-(H2O)2(H3O)+(H2O)3I-]. [ABSTRACT FROM AUTHOR]

A computer program has been written to perform analytic derivative calculations of magnetic response properties based on uncontracted multireference configuration interaction (MRCI) wavefunctions. At this point, orbital relaxation is neglected in this treatment and the basis functions are assumed to be independent of the perturbation. It is therefore equivalent to an untruncated sum-over-states (SOS) approach to the g-tensor. Hence, the treatment is more rigorous than all previous MRCI g-tensor calculations that have used truncated SOS formulations. In the present calculations, MRCI calculations were performed on top of full valence complete-active space self-consistent field (CASSCF) reference wavefunctions without truncation of the reference or correlation spaces. Results are reported for two types of spin-orbit coupling operators, a simple effective nuclear charge approximation and the spin-orbit mean field (SOMF) approximation. The effects of orbital relaxation are discussed. The values obtained can be used in future calibration studies. [ABSTRACT FROM AUTHOR]

An implementation of the projected gradient method for locating the minimum energy crossing point between electronic states of different symmetry/multiplicity within the equation-of-motion coupled-cluster family of methods is reported. The method is applied to characterize the intersections between electronic states in [image omitted], NO2, and para-benzyne using the excitation energies, ionization potential, and spin-flip variants of the equation-of-motion coupled-cluster methods. The performance of the algorithm is discussed and recommendations for improving the convergence in problematic situations are given. [ABSTRACT FROM AUTHOR]

Analytic energy gradients with respect to atomic coordinates for systems with translational invariance are formulated within the framework of Kohn-Sham Density Functional Theory. The energy gradients are implemented in the BAND program for periodic DFT calculations which directly employs a Bloch basis set made up of Slater-type (STOs) and numeric atomic orbitals (NAOs). The details of our implementation are described including the use of symmetry in the reciprocal and direct spaces, as well as the application of the frozen core approximation. [ABSTRACT FROM AUTHOR]

In this work, we present the analytical gradient of dual-basis second-order Møller-Plesset perturbation theory within the resolution-of-the-identity approximation (DB-RI-MP2). Interestingly, analytical DB-RI-MP2 gradient theory involves significant changes to both the theory and computation of the coupled-perturbed self-consistent field equations (CPSCF). From a theoretical point of view, the number of orbital responses required in DB-RI-MP2 analytical gradient theory has been reduced to the product of the number of occupied and virtual orbitals determined by the rank of the small atomic orbital (AO) basis. From a computational point of view, the DB-CPSCF equations can be solved within this smaller space at a fraction of the computational cost. Additional computational savings can be obtained during the digestion of the four-centered AO integral derivatives and the efficient underlying DB-SCF procedure, which lead to a significant overall reduction in the computational cost necessary for treating molecular systems containing less than 100 atoms. Based on stringent chemical tests and a detailed computational timings analysis, it was found that DB-RI-MP2 reproduces molecular equilibrium structures with an accuracy that approximates RI-MP2 using the target AO basis but at a dramatically reduced cost, thereby enabling more routine use of large AO basis sets during geometry optimizations at the MP2 level of theory. [ABSTRACT FROM AUTHOR]

The optimized geometries, total energies, and vibrational frequencies of the polyacenes and perfluoropolyacenes in their closed-shell singlet and lowest triplet states have been studied using the B3LYP method in conjunction with double-ζ plus polarization (DZP) basis sets. The equilibrium structures of all the polyacenes and perfluoropolyacenes are planar, with D2h symmetry, and the singlet and triplet geometries display interesting patterns for each family of molecules. The largest singlet-triplet structural changes appear in the outermost C-C distances. The small perfluoropolyacene systems have ground state singlet states, and the singlet-triplet energy gaps decrease as the number of linearly fused benzene rings increases. With the number of rings greater than seven, the triplet state of the perfluorinated species falls below the closed-shell singlet. For the parent polyacenes, the same shift occurs between n = 8 and n = 9. Correspondingly, the LUMO-HOMO gap for the singlet state decreases with increasing number of linearly fused perfluoro benzene rings. The predicted singlet-triplet separations for the perfluorinated compounds range from 54 kcal mol-1 (perfluoronaphthalene) to -7 kcal mol-1 (n = 10). [ABSTRACT FROM AUTHOR]

Natural localized molecular orbitals (NLMOs) have been used in local correlation calculations. For a test set of 30 molecules it is shown that the results obtained with NLMOs and Pipek-Mezey localization are very similar. However, NLMOs are much less sensitive to the basis set, in particular when diffuse functions are used. Based on natural population analysis (NPA) a new method to determine the domains in local correlation methods is proposed. It is demonstrated that this yields domains that are very insensitive to the choice of the basis set. For all 30 molecules the same domains are obtained with six different basis sets ranging from cc-pVDZ to aug-cc-pVQZ. This allows one to define the local correlation methods more uniquely than with previous methods. [ABSTRACT FROM AUTHOR]