Electrostatic effects on conformational equilibria: Solvation enthalpies and the reaction field theory (original) (raw)
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Conformational analysis of 2-cyano-1,1-dihydroxyethane in solution
Journal of Physical Organic Chemistry, 1996
The conformational space of the 2-cyano-1,l-dihydroxyethane molecule was studied at the semi-empirical PM3 level and ab initio MP2/6-31G**//6-31G level in the gas phase and in a low-polarity medium. This system has been chosen as a model compound for 2-cyanocyclohexanone propylene and ethylene acetals. This has allowed the study of the role of polar groups on the relative conformation of two adjacent OH groups, which is of interest also in relation to the anomeric effect in carbohydrate chemistry. Solvent effects are taken into account using a continuum model with general cavity shapes,
Conformational equilibria of α-substituted carbonyl compounds. Study of solvent effects
Journal of Molecular Structure: THEOCHEM, 1993
PM3 computations have been performed to study conformational equilibria of a series of a-substituted carbonyl compounds in solution. The solvent is taken into account in the calculations by using cavity models. The compounds studied are a series of substituted acetaldehydes CH,R-CHO and cyclohexanones C,H,OR (R = F, Cl, Br, CN, NO,). The predicted solvent effects are in agreement with available experimental data. The discussion emphasizes the role played by the multipole moments and the necessity of going beyond the ellipsoidal cavity shape approximation in the case of substituted cyclohexanones.
The Raman and infrared spectra of solid methyl-5-amino-4-cyano-3-(methylthio)-1H-pyrazole-1- carbodithioate (MAMPC, C7H8N4S3) were measured in the spectral range of 3700–100 cm−1 and 4000–200 cm−1 with a resolution of 4 and 0.5 cm−1, respectively. Room temperature 13C NMR and 1H NMR spectra from room temperature down to −60 ◦C were also recorded. As a result of internal rotation around C–N and/or C–S bonds, eighteen rotational isomers are suggested for the MAMPC molecule (Cs symmetry). DFT/B3LYP and MP2 calculations were carried out up to 6-311++G(d,p) basis sets to include polarization and diffusion functions. The results favor conformer 1 in the solid (experimentally) and gaseous (theoretically) phases. For conformer 1, the two –CH3 groups are directed towards the nitrogen atoms (pyrazole ring) and C S, while the –NH2 group retains sp2 hybridization and C–C N bond is quasi linear. To support NMR spectral assignments, chemical shifts (ı) were predicted at the B3LYP/6- 311+G(2d,p) level using the method of Gauge-Invariant Atomic Orbital (GIAO) method. Moreover, the solvent effect was included via the Polarizable Continuum Model (PCM). Additionally, both infrared and Raman spectra were predicted using B3LYP/6-31G(d) calculations. The recorded vibrational, 1H and 13C NMR spectral data favors conformer 1 in both the solid phase and in solution. Aided by normal coordinate analysis and potential energy distributions, confident vibrational assignments for observed bands have been proposed. Moreover, the CH3 barriers to internal rotations were investigated. The results are discussed herein are compared with similar molecules whenever appropriate.
bpl.turkfizikdernegi.org
The optimized structural parameters (bond lengths and bond angles), electronic, NLO, and spectroscopic properties (FT-IR, UV 1 H and 13 C NMR) of 5-bromo-1-(2-cyano-pyridin-4-yl)-1H-indazole-3-carboxylic acid diethylamide were investigated by the B3LYP, B3PW91, and BPV86 level of theory with the 6-311++G(d,p) basis set and were compared with the experimental values. The characterization of the covalent bond was carried out with the Wiberg bond indices (WBIs) derived through the natural bond orbital analysis. The complete vibrational frequencies have been made on the basis of the potential energy distribution (PED) obtained by the Vibrational Energy Distribution Analysis 4 (VEDA4) program. Moreover, the electronic properties, such as the energies of the frontier molecular orbitals (HOMO and LUMO), energy gap (∆ELUMO−HOMO), global reactivity descriptors (global hardness, global softness, chemical potential, and electrophilicity index), absorption wavelengths (λ), excitation energies (E) and oscillator strengths (f) were performed by the time-dependent density functional theory (TD-DFT) in the chloroform solvent and gas phase. The major contributions to the electronic transitions for the UV-Vis analysis were obtained using the Gauss-Sum 2.2 program Solvent effects on NMR tensors of the title compound have been investigated at the B3LYP, B3PW91, and BPV86 levels using the chloroform solvent and gas phase The weak interaction (Van der Waals), strong attraction (hydrogen bond) and strong repulsion (steric effect) in the title compound were analyzed via the reduced density gradient (RDG) analysis using Multiwfn software.
2014
The system of 1,2-disubstituted-1,2-diphenylethanes is characterized by the strong preference of the conformation with antiperiplanar phenyl groups in both diastereomers which positions the other substituents ap in the erythro and (+)-sc in the threo isomer. This proved true for the isomers of 3-amino-2,3-diphenylpropanoic acid and its N-acetyl and ester derivatives as evidenced by J 2H3H-couplings above 10 Hz found in formamide. Only the zwitterion of erythro amino acid 1a showed a smaller value of 9 Hz indicating charge attraction overcoming partly the steric interactions. Molecular mechanics calculations by means of the Scheraga force field corroborated the interpretation. An IR-study of the equilibrium zwitterionneutral amino acid showed the latter to be preferred in aprotic solvents ≈ 100% in pure DMSO for 1a but decreased in the presence of water, until only the zwitterion was detected in 80% DMSO. The J 2H3Hcouplings of 1a (10.3 Hz in D 2 O and 7.7.Hz in pure DMSO) indicated that solvation by water increases steric hindrance and suggested a strong hydrogen bond CO 2 H….NH 2 in aprotic media. The pK-values of all compounds were determined potentiometricly in 80% methylcellosolve and in 90% DMSO. The pK 1 's of threo zwitterion 1a for COOH were 0.8 pK units larger than those of the erythro isomer as predicted for anti vs. gauche charged groups. The pK 2values of the isomers do not differ significantly, the electrostatic effects are apparently offset by steric hindrance from solvation of NH 3 +. The pK's of the ester and N-acetylated derivatives differ from those of the zwitterion by ca. 2 pK units evidencing strong effects of charge interaction. In organic solvents the acidities of COOH and NH 3 + change strongly in opposite directions leading to appearance of neutral amino acid bands proven by the IR data.
Polarization-corrected electrostatic potentials of aromatic compounds
Journal of the American Chemical Society, 1993
The electrostatic potentials (EPS) corrected for polarization (TPS) of the aromatic compounds benzene, aniline, chlorobenzene, nitrobenzene, phenol, benzamide, and N-phenylacetamide have been calculated at the ab initio SCF level within three basis sets: 6-31G**, MINI-1, and STO-3G. For chlorobenzene in its MINI-1-optimized geometry, the calculation was also performed within MINI-1**. By reference to 6-31G**, the MINI-1-computed EP is much more satisfactory than the STO-3G-computed EP, whereas the MINI-1 and STO-3G basis sets give very similar total potentials corrected for polarization (TPs). The MINI-1** basis set appears to be miscalibrated for computing EPs. It provides qualitative results that differ from those obtained with the 6-31G** basis set. The EP has a negative well above the middle of the benzene ring, while the TP exhibits a negative crown just above the benzene carbon atoms, where electrophilic attack takes place. The TP calculated for the interaction of nitrobenzene with a hydride ion instead of a proton allowed analyzation of the effects of polarization on the positive EP above the N-C bond.
Journal of Physical Chemistry A, 1997
The global conformational potentials of 1,2-ethanediol, 1,2-ethanediamine, and 2-aminoethanol (X-CH 2-CH 2-Y; X, Y) OH or NH 2) were obtained at the MP2/6-311+G(2d,p) level by scanning through the dihedral angles of the two functional groups and the carbon-carbon bond with the remaining nuclear coordinates being energy-minimized. It was found that the potentials could be represented by the direct-bond potentials between the adjacent molecular fragments and by the through-space electrostatic potentials between the vicinal and geminal fragments. Here, the through-direct-bond potentials are represented by the conventional three Fourier terms of the internal rotation angles, and the through-space potentials, which include the intramolecular hydrogen bonding between X and Y, are represented by the general functional forms of the electric dipoledipole, dipole-quadrupole, and quadrupole-quadrupole interaction terms. The fitted electrostatic interaction strengths between the X and Y fragments are in good agreement with the predictions of the theoretical molecular fragment dipole and quadrupole moments calculated by the Hirshfeld charge population analysis. Under the present energy decomposition scheme, the intrinsic gauche interactions, which are free of the contribution of the intramolecular H-bonding, could be obtained and correlated with the group electronegativities of X and Y. The potentials were also calculated by the MM3 molecular mechanics method and compared with the present results. With the global conformation potentials, the thermodynamic functions of the molecules and also their individual conformers are calculated and compared with the gas-phase experimental thermodynamic data in the literature.