Ljupčo Pejov

Bio: Ljupčo Pejov is an academic researcher from Macedonian Academy of Sciences and Arts. The author has contributed to research in topics: Anharmonicity & Ab initio. The author has an hindex of 21, co-authored 94 publications receiving 1149 citations. Previous affiliations of Ljupčo Pejov include Uppsala University.

Model for photoinduced bending of slender molecular crystals.

Abstract: 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 cons...

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

Abstract: 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...

Probing Structural Perturbation in a Bent Molecular Crystal with Synchrotron Infrared Microspectroscopy and Periodic Density Functional Theory Calculations.

Abstract: The range of unit cell orientations generated at the kink of a bent single crystal poses unsurmountable challenges with diffraction analysis and limits the insight into the molecular-scale mechanism of bending. On a plastically bent crystal of hexachlorobenzene, it is demonstrated here that spatially resolved microfocus infrared spectroscopy using synchrotron radiation can be applied in conjunction with periodic density functional theory calculations to predict spectral changes or to extract information on structural changes that occur as a consequence of bending. The approach reproduces well the observed trends, such as the wall effects, and provides estimations of the vibrational shifts, unit cell deformations, and intramolecular parameters. Generally, expansion of the lattice induces red-shift while compression induces larger blue-shift of the characteristic ν(C–C) and ν(C–Cl) modes. Uniform or non-uniform expansion or contraction of the unit cell of 0.1 A results in shifts of several cm–1, whereas def...

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

Abstract: 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 calculation of vibrational absorption and circular dichroism spectra using density functional force fields

Abstract: : The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Red-, blue-, or no-shift in hydrogen bonds: a unified explanation.

Abstract: We provide a simple explanation for X-H bond contraction and the associated blue shift and decrease of intensity in IR spectrum of the so-called improper hydrogen bonds This explanation organizes hydrogen bonds (HBs) with a seemingly random relationship between the X-H bond length (and IR frequency and its intensity) to its interaction energy The factors which affect the X-H bond in all X-H [midline ellipsis] Y HBs can be divided into two parts: (a) The electron affinity of X causes a net gain of electron density at the X-H bond region in the presence of Y and encourages an X-H bond contraction (b) The well understood attractive interaction between the positive H and electron rich Y forces an X-H bond elongation For electron rich, highly polar X-H bonds (proper HB donors) the latter almost always dominates and results in X-H bond elongation, whereas for less polar, electron poor X-H bonds (pro-improper HB donors) the effect of the former is noticeable if Y is not a very strong HB acceptor Although both the above factors increase with increasing HB acceptor ability of Y, the shortening effect dominates over a range of Ys for suitable pro-improper X-Hs resulting in a surprising trend of decreasing X-H bond length with increasing HB acceptor ability The observed frequency and intensity variations follow naturally The possibility of HBs which do not show any IR frequency change in the X-H stretching mode also directly follows from this explanation

Advanced Engineering Mathematics. By C. R. Wylie. Pp. xiv, 813. 1966. (McGraw-Hill.)

Abstract: Ordinary differential equations of the first order linear differential equations complex numbers and linear algebra simultaneous linear differential equations numerical methods the descriptive theory of nonlinear differential equations mechanical systems and electric circuits Fourier series Fourier integrals and Fourier transforms the Laplace transformation partial differential equations Bessel functions and Legendre polynomials applications and further properties of matrices vector analysis the calculus of variations analytic functions of a complex variable infinite series in the complex plane the theory of residues conformal mapping.

Mechanically Responsive Molecular Crystals

Abstract: Pancě Naumov,*,† Stanislav Chizhik,‡,§ Manas K. Panda,† Naba K. Nath,† and Elena Boldyreva*,‡,§ †New York University Abu Dhabi, P.O. Box 129188, Abu Dhabi, United Arab Emirates ‡Institute of Solid State Chemistry and Mechanochemistry, Siberian Branch of Russian Academy of Sciences, ul. Kutateladze, 18, Novosibirsk 630128, Russia Novosibirsk State University, ul. Pirogova, 2, Novosibirsk 630090, Russia

Water structural transformation at molecular hydrophobic interfaces

Abstract: Hydrophobic hydration is considered to have a key role in biological processes ranging from membrane formation to protein folding and ligand binding. Historically, hydrophobic hydration shells were thought to resemble solid clathrate hydrates, with solutes surrounded by polyhedral cages composed of tetrahedrally hydrogen-bonded water molecules. But more recent experimental and theoretical studies have challenged this view and emphasized the importance of the length scales involved. Here we report combined polarized, isotopic and temperature-dependent Raman scattering measurements with multivariate curve resolution (Raman-MCR) that explore hydrophobic hydration by mapping the vibrational spectroscopic features arising from the hydrophobic hydration shells of linear alcohols ranging from methanol to heptanol. Our data, covering the entire 0-100 °C temperature range, show clear evidence that at low temperatures the hydration shells have a hydrophobically enhanced water structure with greater tetrahedral order and fewer weak hydrogen bonds than the surrounding bulk water. This structure disappears with increasing temperature and is then, for hydrophobic chains longer than ~1 nm, replaced by a more disordered structure with weaker hydrogen bonds than bulk water. These observations support our current understanding of hydrophobic hydration, including the thermally induced water structural transformation that is suggestive of the hydrophobic crossover predicted to occur at lengths of ~1 nm (refs 5, 9, 10, 14).