Ljupčo Pejov

On the nature of blueshifting hydrogen bonds: Ab initio and density functional studies of several fluoroform complexes

Potential energy hypersurfaces (PESs) for four fluoroform complexes (with acetonitrile, ethyleneoxide, formaldehyde, and water) were explored at the HF, MP2, and B3LYP/6-311++G(d,p) levels of theory. Anharmonic C–H stretching vibrational frequency shifts are reported for all minima located on the studied PESs. In all cases, the lowest-energy minimum occurs for a C–H⋯O(N) hydrogen-bonded arrangement and is characterized by a significant C–H frequency blueshift (upshift), while additional minima [for “reversed” orientations, in which there is no direct C–H⋯O(N) contact] show only small C–H frequency upshifts. The large blueshifts found for the hydrogen-bonded arrangements are predominantly caused by the electronic exchange interaction, as revealed by Kitaura–Morokuma (KM) analysis, while the purely electrostatic+polarization interaction leads to C–H frequency redshifts, which was proven both by the KM analysis and the charge field perturbational (CFP) approach. The large net blueshifting effect of the excha...

Model for Photoinduced Bending of Slender Molecular Crystals

The growing realization that photoinduced bending of slender photoreactive single crystals is surprisingly common has inspired researchers to control crystal motility for actuation. However, new mechanically responsive crystals are reported at a greater rate than their quantitative photophysical characterization; a quantitative identification of measurable parameters and molecular-scale factors that determine the mechanical response has yet to be established. Herein, a simple mathematical description of the quasi-static and time-dependent photoinduced bending of macroscopic single crystals is provided. This kinetic model goes beyond the approximate treatment of a bending crystal as a simple composite bilayer. It includes alternative pathways for excited-state decay and provides a more accurate description of the bending by accounting for the spatial gradient in the product/reactant ratio. A new crystal form (space group P21/n) of the photoresponsive azo-dye Disperse Red 1 (DR1) is analyzed within the constraints of the aforementioned model. The crystal bending kinetics depends on intrinsic factors (crystal size) and external factors (excitation time, direction, and intensity).

A gradient-corrected density functional study of indole self-association through N–H⋯π hydrogen bonding

Abstract A B3LYP/6-31++G(d,p) study of indole dimer was performed. The optimized geometry reveals the existence of N–H⋯π hydrogen bond in which the benzenoid ring of one subunit acts as a proton acceptor, the interplanar angle between the two monomeric units being 89.4° (a T-shaped structure), with the center-of-mass separation of 6.207 A. The counterpoise-corrected interaction energy is 2.15 kcal mol −1 (9.00 kJ mol −1 ) . Anharmonic vibrational frequencies of monomeric and dimeric N–H oscillators, their change upon dimerization and the intensity enhancement are excellently reproduced by one-dimensional B3LYP/6-31++G(d,p) vibrational potentials.

Structure and Dynamics of Water Dangling OH Bonds in Hydrophobic Hydration Shells. Comparison of Simulation and Experiment

Molecular dynamics and electric field strength simulations are performed in order to quantify the structural, dynamic, and vibrational properties of non-H-bonded (dangling) OH groups in the hydration shell of neopentane, as well as in bulk water. The results are found to be in good agreement with the experimentally observed high-frequency (∼3660 cm(-1)) OH band arising from the hydration shell of neopentanol dissolved in HOD/D(2)O, obtained by analyzing variable concentration Raman spectra using multivariate curve resolution (Raman-MCR). The simulation results further indicate that hydration shell dangling OH groups preferentially point toward the central carbon atom of neopentane to a degree that increases with the lifetime of the dangling OH.

Ab initio HF, density functional and experimental studies on the IR spectra and structure of 1,2-benzisothiazol-3-(2H)-thione-1,1-dioxide (thiosacchanin) and its nitranion.

The spectral and structural changes taking place in the course of the conversion of 1,2-benzisothiazol-3-(2H)-thione-1,1-dioxide (thiosaccharin) into a nitranion have been studied on the basis of both IR spectra and ab initio HF 6-31G(d) and BLYP 6-31G(d,p) force field calculations. The conversion causes nu(as)SO2 and nu(s)SO2 frequency decreases of 47 and 13 cm(-1), respectively, and other spectral changes. The nuC=S coordinate is strongly delocalized. The ab initio geometries of the isolated molecule and nitranion agree well with the single-crystal X-ray ones, determined for thiosaccharin and its sodium (potassium) monohydrate salts, respectively. The nitranionic charge is delocalized almost uniformly within the thiocarbonyl (0.29 e-), sulfonyl (0.24 e-), and phenylene (0.24 e-) groups, and the nitranionic center (0.23 e-).

Experimental and quantum chemical study of pyrrole self-association through N–H⋯π hydrogen bonding

An FT-IR study of pyrrole self-association in CCl4 solutions was carried out. According to the IR measurements, pyrrole forms self-associated dimeric species via N– H· · ·p hydrogen bonding. This was also confirmed by quantum chemical calculations for pyrrole monomer and dimer at B3LYP/6-31 þþ G(d,p) level of theory. A T-shaped minimum was located on B3LYP/6-31þþG(d,p) PES of pyrrole dimer characterized with a hydrogen bond of an N– H· · ·p type, with centers-of-mass separation of monomeric units of 4.520 A u ,H · ·· p distance of 2.475 A u , the interplanar angle between the two monomeric units being 72.98. The anharmonic vibrational frequency shift upon dimer formation calculated on the basis of 1D DFT vibrational potentials is in excellent agreement with the experimental data (84 vs. 87 cm 21 ). Harmonic vibrational analysis predicts somewhat smaller shift (68 cm 21 ). On the basis of NIR spectroscopic data, anharmonicity constants for the 2n(N –H) and 2n(N– H· · ·p) vibrational transitions were calculated. The orientational dynamics of monomeric and self-associated pyrrole species was studied within the framework of the transition dipole moment time correlation function formalism. The period of essentially free rotation in the condensed phase reduces from 0.05 ps for the monomeric pyrrole to 0.02 ps for the proton-donor molecule within the dimer. q 2003 Published by Elsevier Science B.V.

The O–H⋯π hydrogen bonded phenol–benzene(+) radical cationic dimer: a gradient-corrected density functional study

Abstract A B3LYP and mPW1PW91/6-31++G( d , p ) study of phenol–benzene(+) radical cation was performed, revealing an existence of T-shaped O–H⋯π hydrogen bonded minima at both PES-s, with center-of-mass separation of 5.087 A (B3LYP) and 4.970 A (mPW1PW91) and the interplanar angle between monomeric units of 89.9° and 89.4° (at B3LYP and mPW1PW91 levels correspondingly). Calculated anharmonic O–H vibrational frequencies on the basis of one-dimensional DFT vibrational potentials reproduce excellently the experimentally measured ν (OH) frequency shift upon this interaction. According to CFP-like calculations, most of the interaction energy (47.09 and 49.64 kJ mol −1 at B3LYP and mPW1PW91 levels correspondingly) is of electrostatic origin.

The O–H stretching vibrations in the hydrogen bonded (PHENOL)2+ radical cationic dimer. A gradient-corrected hybrid Hartree–Fock-density functional study

Abstract The global minimum on B3LYP, mPW1PW91 and PBE1PBE/6-31++G(d,p) potential energy surfaces (PESs) of the (phenol)2+ cationic radical dimer corresponds to O–H+⋯O hydrogen-bonded structure, with an additional, although much weaker C–H⋯O hydrogen bond, as revealed by AIM analysis. Excellent agreement with experimental data is obtained for the anharmonic vibrational frequency shift of the dangling O–H oscillator on the basis of one-dimensional DFT O–H stretching potentials. However, theoretical calculations suggest that the ν(O–H+⋯O) mode due to the hydrogen-bonded O–H oscillator should appear at significantly lower frequencies than it was first estimated on the basis of experimental dissociation spectroscopy combined with an ion trap technique data.

Vibrational anharmonicity and vibrational Stark effect of sulfate ions trapped in potassium, rubidium and cesium chromate

Room and low temperature (approximately 100 K) FT-IR and Raman spectra of the sulfate doped K2CrO4, Rb2CrO4 and Cs2CrO4 were recorded. The positions of the nu1, nu3, and nu4 fundamental mode components of the dopant anions were measured. Nine (out of possible ten) second-order stretch-stretch vibrational transitions of the dopant anions were detected. On the basis of these data, the anharmonicity constants and the corresponding harmonic eigenvalues were calculated for several vibrational transitions using second-order perturbation theory expressions. The anharmonicity of the studied second-order transitions of the type nu1 + nu3i increases in the order (SO4/K2CrO4) < (SO4/Rb2CrO4) < (SO4/Cs2CrO4), while for those of the type nu3j + nu3i it basically follows the trend: (SO4/K2CrO4) > (SO4/Rb2CrO4) < (SO4/Cs2CrO4). The measured relative Stark splittings of the nu3 and nu4 mode components of the dopant SO4(2-) anions, as well as the average X13i/3i3j values decrease in the order (SO4/K2CrO4) > (SO4/Rb2CrO4) > (SO4/Cs2CrO4). In all cases, the splitting is larger for nu3 than for nu4 modes, indicating a smaller angular than the bond length distortion. The theory of vibrational Stark effect suggests that the observed frequency shifts of the nu1, nu3 and nu4 mode components may be attributed to the increase of the field strength at the doped anion site going from K2CrO4 to Cs2CrO4. The Stark splitting of these modes, on the other hand, implies that the internal crystalline field vector is almost parallel to the (hypothetical) C2 axis of the slightly distorted dopant tetrahedral anions.

Al3+, Ca2+, Mg2+, and Li+ in aqueous solution: calculated first-shell anharmonic OH vibrations at 300 K.

The anharmonic OH stretching vibrational frequencies, ν(OH), for the first-shell water molecules around the Li(+), Ca(2+), Mg(2+), and Al(3+) ions in dilute aqueous solutions have been calculated based on classical molecular dynamics (MD) simulations and quantum-mechanical (QM) calculations. For Li(+)(aq), Ca(2+)(aq), Mg(2+)(aq), and Al(3+)(aq), our calculated IR frequency shifts, Δν(OH), with respect to the gas-phase water frequency, are about -300, -350, -450, and -750 cm(-1), compared to -290, -290, -420, and -830 cm(-1) from experimental infrared (IR) studies. The agreement is thus quite good, except for the order between Li(+) and Ca(2+). Given that the polarizing field from the Ca(2+) ion ought to be larger than that from Li(+)(aq), our calculated result seems reasonable. Also the absolute OH frequencies agree well with experiment. The method we used is a sequential four-step procedure: QM(electronic) to make a force field+MD simulation+QM(electronic) for point-charge-embedded M(n+) (H(2)O)(y) (second shell) (H(2)O)(z) (third shell) clusters+QM(vibrational) to yield the OH spectrum. The many-body Ca(2+)-water force-field presented in this paper is new. IR intensity-weighting of the density-of-states frequency distributions was carried out by means of the squared dipole moment derivatives.