Tom Sundius

Scaling of ab initio force fields by MOLVIB

Abstract Nowadays, it is possible to perform ab initio calculations on relatively large molecules. The vibrational frequencies obtained in this way differ from the observed frequencies, partly because of anharmonicity, and partly because of approximations in the quantum mechanical methods used (neglect of electron correlation and basis set truncation). Because of this, ab initio force fields generally have to be scaled using spectroscopic data. A new version of MOLVIB, a program for vibrational force field calculations, has recently been prepared. With this program, it is now possible to perform scale factor calculations according to several methods, which are described. Results of application to some small molecules are also presented.

A new damped least-squares method for the calculation of molecular force fields

Abstract A modified Gauss method for the calculation of molecular force fields from spectroscopic data is presented. Comparison with other methods shows that it is capable of overcoming many of the frequently occurring convergence problems in least-squares force field optimization. The method has been tested by calculating valence and Urey-Bradley force fields for formaldehyde. The results are in good agreement with other force field calculations on the same molecule.

Competitive ring hydride shifts and tolyl-benzyl rearrangements in tolyl and silatolyl cations

Abstract Transition states for two plausible isomerization pathways of tolyl and silatolyl cations are predicted at the B3LYP/6-31(d,p) level of theory. The barrier heights for the interconversion of the ortho- and meta- isomers are high for both tolyl (50 kcal/mol) and silatolyl (57 kcal/mol) cations. Competitive hydride shifts from methyl and silyl groups to the positive charge center at the phenyl ring leading to benzyl and silabenzyl cations have sufficiently different barrier heights: the one for tolyl cation (26 kcal/mol) is close to the upper excess energy limit of the nucleogenic tolyl cation, while this barrier for the silatolyl cation is substantially lower (9 kcal/mol). Stationary points for the path leading from the benzyl cation to the global minimum of the C 7 H 7 + system, i.e. the tropylium cation, were also found.

Vibrational spectra, ab initio/DFT electronic structure calculations, and normal coordinate analysis of 2-bromo-5-fluorobenzaldehyde

FT-IR (4000-400 cm(-1)) and FT-Raman (3500-50 cm(-1)) spectral measurements of solid samples of 2-bromo-5-fluorobenzaldehyde (BFB) have been done. Ab initio (RHF/6-311G*) and DFT (B3LY/6-311G* and B3PW91/6-311G*) calculations have been performed giving energies, optimized structures, harmonic vibrational frequencies, depolarization ratios, infrared intensities, Raman activities and atomic displacements. Furthermore force field calculations have been performed by normal coordinate analysis. Force field calculations showed that several normal modes are mixed in terms of the internal coordinates. A complete assignment of the observed spectra, based on spectral correlations, electronic structure and normal coordinate analysis, has been proposed. Optimization leads to C(S) symmetry with O-trans and O-cis isomers, with respect to aldehydic oxygen and bromine, with O-trans-isomer as the low energy stable form. The energy difference between the two isomers is 2.95084 kcal/mol. The results of the calculations have been used to simulate IR and Raman spectra for BFB that showed excellent agreement with the observed spectra. The SQM method, which implies multiple scaling of the ab initio and DFT force fields has been shown superior to the uniform scaling approach.

Crystal structure, Hirshfeld surfaces and DFT computation of NLO active (2E)-2-(ethoxycarbonyl)-3-[(1-methoxy-1-oxo-3-phenylpropan-2-yl)amino] prop-2-enoic acid

Nonlinear optical (NLO) activity of the compound (2E)-2-(ethoxycarbonyl)-3-[(1-methoxy-1-oxo-3-phenylpropan-2-yl)amino] prop-2-enoic acid is investigated experimentally and theoretically using X-ray crystallography and quantum chemical calculations. The NLO activity is confirmed by both powder Second Harmonic Generation (SHG) experiment and first hyper polarizability calculation. The title compound displays 8 fold excess of SHG activity when compared with the standard compound KDP. The gas phase geometry optimization and vibrational frequencies calculations are performed using density functional theory (DFT) incorporated in B3LYP with 6-311G++(d,p) basis set. The title compound crystallizes in non-centrosymmetric space group P21. Moreover, the crystal structure is primarily stabilized through intramolecular N-H···O and O-H···O hydrogen bonds and intermolecular C-H···O and C-H···π interactions. These intermolecular interactions are analyzed and quantified using Hirshfeld surface analysis and PIXEL method. The detailed vibrational assignments are performed on the basis of the potential energy distributions (PED) of the vibrational modes.

Ab initio/DFT electronic structure calculations, spectroscopic studies and normal coordinate analysis of 2-chloro-5-bromopyridine.

FT-IR (4000-400 cm(-1)) and FT-Raman (3500-50 cm(-1)) spectral measurements of solid sample of 2-chloro-5-bromopyridine have been done. Ab initio and DFT calculations have been performed giving energies, optimized structures, harmonic vibrational frequencies, depolarization ratios, IR intensities, Raman activities and atomic displacements. Furthermore, force field calculations have been performed by normal coordinate analysis. A complete assignment of the observed spectra, based on spectral correlations, electronic structure calculations and normal coordinate analysis, has been proposed. The results of the calculations have been used to simulate IR and Raman spectra for the molecule that showed good agreement with the observed spectra. The SQM method, which implies multiple scaling of the DFT force fields, has been shown superior to the uniform scaling approach. The energy and oscillator strength calculated by Time-dependent DFT results are in good agreement with the experimental results.

Vibrational spectra of silatranes and germatranes XM(OCH2CH2)3N (X = F,Cl,H; M = Si,Ge). The problem of the theoretical prediction of condensed phase spectra

The structures of silatranes and germatranes XM(OCH(2)CH(2))(3)N (X=F,Cl,H; M=Si,Ge) were optimized and their vibrational spectra were calculated at the B3LYP/aug-cc-pVDZ level of theory. Theoretical frequencies of vibrations perpendicular to the C(3) axis (E type) are in good agreement with experimental values, while the axial vibrations (MX and M...N stretchings) demonstrate a significant discrepancy with experimental spectra recorded for the crystalline state. This discrepancy stems from the well-known difference in the MX and M...N bond lengths in gas and solid state. The force constant scaling procedure was used to compensate for this difference. As a result a set of scaling factors was refined for 1-Cl-germatrane (the unique atrane for which the distinction between A and E modes was experimentally established). This set was transferred to the theoretical force fields of other atranes, which provided a fair reproduction of their experimental frequencies. The analysis of the normal modes allowed us to assign the ν M...N mode to bands in the 180-270 cm(-1) frequency range, although large contributions of these coordinates are in two other modes in the 450-500 cm(-1) and 600-800 cm(-1) frequency ranges. The frequencies of degenerate vibrations (with vectors perpendicular to the C(3) axis) do not depend substantially on the axial atom (X and M) substitution, while those of A-type in the 200-700 cm(-1) frequency range vary significantly.

Molecular structural, non-linear optical, second order perturbation and Fukui studies of Indole-3-Aldehyde using density functional calculations

Indole-3-Aldehyde is a new organic non-linear material having good second harmonic generation. The optimized molecular geometry, harmonic vibrational frequencies, infrared intensities of Indole-3-Aldehyde (I3A, C9H7NO) in the ground state were carried out by using density functional theory (B3LYP) method with 6-31G(d,p) basis set. A detailed interpretation of the infrared spectrum of Indole-3-Aldehyde is reported. The vibrational frequencies are calculated and compared with experimental FT-IR spectra. The theoretical spectrograms of FT-IR of the title compound have been constructed in addition, theoretical information like ONIOM, potential energy surface, NBO, and Fukui function are also calculated. Unambiguous vibrational assignment of all the fundamentals was made using the potential energy distribution.

Vibrational analyses of 1,3-dibenzoyl-4,5-dihydro-1H-imidazole-2-thione and 1,3-dibenzoyl tetrahydropyrimidine-2(1H)-thione by normal coordinate treatment

Vibrational analyses of 1,3-dibenzoyl-4,5-dihydro-1H-imidazole-2-thione and 1,3-dibenzoyl tetrahydropyrimidine-2(1H)-thione were carried out using normal coordinate analysis. FT-IR spectra were recorded in the region 4000-400 cm(-1). The harmonic vibrational frequencies, molecular geometry and atomic charges have been computed, and NBO analysis has been carried out with the help of B3LYP density functional theory (DFT). The computed geometrical bond lengths and bond angles agree well with the crystallographic data. Atomic charges based on Mulliken population analysis, natural population analysis, Hirshfeld-I analysis and CHelpG analysis were calculated using the basis sets of 6-31G(*) and 6-31G(**). Stabilities of the two molecules were analyzed by means of natural bond orbital (NBO) analysis and delocalized π-π(*) interactions.

An experimental and theoretical study of the vibrational spectra and structure of Isosorbide dinitrate

The present work aims at exploring the vibrational spectra of Isosorbide dinitrate and its chemical activity in a five membered ring system. The FT-IR and FT-Raman spectral studies of the Isosorbide dinitrate (ISDN) were carried out. The equilibrium geometry, various bonding features and harmonic vibrational frequencies of ISDN have been calculated using B3LYP density functional theory (DFT) with 6-31G(d,p) as basis set. The calculated HOMO and LUMO energies and density of states (DOS) show the chemical activity of the molecule. Good correlations between the experimental (1)H and (13)C NMR chemical shifts in methanol-d and calculated GIAO shielding tensors were found. The potential energy surface was studied using the DFT method.