Steve Scheiner

Substituent Effects upon Protonation-Induced Red Shift of Phenyl−Pyridine Copolymers

Steve Scheiner et al. — Thu, 15 Dec 2011 16:01:06 PST

Ab initio methods are used to probe the underlying source of the red shift that arises in the absorption band of polymers that contain alternating phenyl and pyridine rings when the N atom of the latter is protonated. It is found that electronic charge transfers from the phenyl to the pyridine as an electron is excited from the HOMO to the LUMO in the protonated system but that no such transfer occurs if the N is unprotonated. Replacement of H atoms by cyano groups reinforces the idea that such an electron-withdrawing substituent facilitates the electron transfer when it is located on the pyridine ring, thereby stabilizing the excited state and thus enhancing the associated red shift; the opposite occurs when cyano is located on the phenyl ring. Unlike cyano and Cl, substitution by F and nitro groups leads to a less consistent pattern of absorption frequencies, suggesting that the effects of these substituents are not simply electrostatic in nature.

The contribution of dispersion to H-bonds between hydrides of first and second-row atoms

M. M. Szczȩśniak et al. — Thu, 15 Dec 2011 16:00:54 PST

H-bonds involving NH₃, PH₃, OH₂, and SH₂ as proton acceptor and HF and HCl as donor, as well as the water dimer, are studied by ab initio molecular orbital methods. The influence of dispersion on the properties of these complexes is calculated by second-order Møller —Plesset perturbation theory using a doubly-polarized double-ξ basis set. This theoretical approach yields H-bond lengths in much better agreement with experiment than distances calculated at the SCF level. The contractions in the bond lengths introduced by dispersion grow rapidly as first-row atoms are replaced with second-row analogs. A similar trend is noted in the H-bond energies where the contribution of dispersion ranges between 22% for the smaller systems and 69% for complexes such as H₂SHCl. In addition, dispersion is seen to be responsible for a large fraction of the stretch in the HX bond resulting from H-bond formation. On the other hand, the angular characteristics of these complexes are relatively unaffected by dispersion but are controlled, instead, primarily by electrostatic factors.

Ab initio investigation of interactions between models of local anesthetics and receptor: Complexes involving amine, phosphate, amide, Na+, K+, Ca2+, and Cl–

Milan Remko et al. — Thu, 15 Dec 2011 16:00:42 PST

Ab initio molecular orbital methods are used to study the interactions between models of local anesthetic molecules and the putative receptors within the nerve membrane: phospholipids and lipoproteins. The tertiary amine terminus of local anesthetics was modeled by ionized and un-ionized trimethylamine, while phosphate monoanion and formamide emulated the appropriate portions of the receptor. The protonated amine forms a very strong complex with the phosphate anion in which the charge is transferred to the phosphate. While somewhat weaker than this, the complex involving the amine (in either its ionized or un-ionized state) and peptide is considerably stronger than interpeptide H bonds, suggesting the anesthetic can disrupt the normal H-bond patterns in a protein. On the other hand, such interactions must compete with the rather tight binding of the anesthetic with the Na⁺, K⁺, Ca²⁺ and Cl^– ions present in vivo.

Hardness and Chemical Potential Profiles for Some Open Shell HAB → HBA Type Reactions. Ab Initio and Density Functional Study

T. Kar et al. — Thu, 08 Sep 2011 14:53:24 PDT

The electronic structure, hardness (η), and chemical potential (μ) for the ¹A‘ and ³A‘‘ states of HNO−HON and the ²A‘‘ state of HSO−HOS have been calculated using HF/6-311++G** and B3LYP/6-311++G** methods. The η and μ profiles of the ¹A‘ state of HNO−HON and those of HSO−HOS are obtained in agreement with the salient features of the maximum hardness principle (MHP). However, a quite erratic η profile is predicted for the ³A‘‘ state of HNO−HON. This can be attributed to the nature of the variation in the energy difference of the two states along the reaction path. The relative energies, ionization potentials (I), and electron affinities (A) are calculated at the stationary points of the B3LYP surface using B3LYP and MPn (Full) methods. Most of these values are obtained in very good agreement with the available experimental data. The η values based on these I and A identify the most stable species correctly but do not follow the expected trend with regard to the relative stability of the transition state. The reason for this anomaly is discussed.

Relativistic Effects in Iron-, Ruthenium-, and Osmium Porphyrins

M.-S. Liao et al. — Thu, 08 Sep 2011 14:53:16 PDT

Site-Site Function and Successive Reaction Counterpoise Calculation of Basis Set Superposition Error for Proton Transfer

A. J. Abkowicz et al. — Thu, 08 Sep 2011 14:53:06 PDT

Factors Contributing to Distortion Energies of Bent H-Bonds. 2. Imine, Carbonyl, Carboxyl and Carboxylate Groups

S. M. Cybulski et al. — Thu, 08 Sep 2011 14:52:58 PDT

The source of the energy requirement for bending a hydrogen bond is sought through decomposition of the total ab initio interaction energy into a number of physically meaningful components. Systems studied include ( H2CNH-H-NH3)+, (H2CO-H-OH2)+, (HCOOH-H-OH2)+, and (HCOO-H-OH)-. As in the earlier cases studied which had been restricted only to small hydride molecules, the Coulombic interaction very closely parallels the change in the total interaction energy as angular distortions are imposed. Despite the larger size of the current molecules and the arbitrariness in selecting an origin for expansion, a multipole series approximation truncated after the R5te rm mimics fairly well the full electrostatic component. The exception to this satisfactory reproduction is the inter-nitrogen H bond of (H2CNH-H-NH3)+. The leading term of the multipole series, encompassing the interaction between the charge of the ionic subunit and the dipole of the neutral, appears capable of providing qualitative guidance, even in these larger systems, as to the shift in proton equilibrium position which accompanies each angular distortion of the H bond.

Extraction of the Principles of Proton Transfers by Ab Initio Methods

Steve Scheiner — Thu, 08 Sep 2011 14:52:48 PDT

Comparison between hydrogen and dihydrogen bonds among H3BNH3, H2BNH2, and NH3

T. Kar et al. — Thu, 08 Sep 2011 14:52:44 PDT

Several possible binary complexes among ammonia-borane, aminoborane, and ammonia, via hydrogen and/or dihydrogen bonds, have been investigated to understand the effect of different hybridization. Møller–Plesset second-order perturbation theory with aug-cc-pVDZ basis set was used. The interaction energy is corrected for basis set superposition error, and the Morokuma–Kitaura method was employed to decompose the total interaction energy. Like H₃BNH₃, the sp² hybridized H₂BNH₂ also participates in H- and dihydrogen bond formation. However, such bonds are weaker than their sp³ analogs. The contractions of BN bonds are associated with blueshift in vibrational frequency and stretches of BH and NH bonds with redshift. The polarization, charge transfer, correlation, and higher-order energy components are larger in dihydrogen bonded complexes, compared to classical H-bonded ammonia dimers.© 2003 American Institute of Physics.

Theoretical vibrational study of the FX⋅⋅⋅O(CH3)2 hydrogen‐bonded complex

Y. Bouteiller et al. — Thu, 08 Sep 2011 14:52:35 PDT

This paper presents the first ab initio attempt to reconstruct the observed band profile of the stretching fundamental v_FX (X=H,D) in the FX⋅⋅⋅O(CH₃)₂ hydrogen‐bonded system. The two‐dimensional potential energy surface V(r_FH,R_F⋅⋅⋅O) is evaluated by means of large basis set SCF calculations. The related force constants up to the fourth order are obtained via the analytical fit to a polynomial expansion. The vibrational problem is solved by means of a variational treatment which includes the effects of mechanical anharmonicity. The side bands of the stretching fundamental v_FX are described in terms of the v_FX ±nv_FX⋅⋅⋅O combination bands in excellent agreement with experiment.

Vibrational Frequencies and Intensities of H-Bonded Systems. 1:1 and 1:2 Complexes of NH3 and PH3 with HFVibrational frequencies and intensities of H‐bonded systems. 1:1 and 1:2 complexes of NH3 and PH3 with HF

I. J. Kurnig et al. — Thu, 08 Sep 2011 14:52:25 PDT

Frequencies and intensities are calculated by ab initio methods for all vibrational modes of the 1:1 H₃X–HF and 1:2 H₃X–HF–HF complexes (X=N,P). The HF stretching frequencies are subject to red shifts, roughly proportional to the strength of the H bond, and to manyfold increases in intensity. Although the intramolecular frequency shifts within the proton acceptors are relatively modest, the intensities of the NH₃ stretches are magnified by several orders of magnitude as a result of H bonding (in contrast to PH₃ which exhibits little sensitivity in this regard). The frequencies and intensities corresponding to bending of the H₃X–HF H‐bond rise with increasing H‐bond strength while the properties of the other intermolecular modes appear somewhat anomalous at first sight. The intensity patterns are analyzed by means of atomic polar tensors which reveal that intensification of the proton donor stretch is chiefly due to increasing charge flux associated with H‐bond formation. The different behavior of the N–H and P–H stretching intensities is attributed to the opposite sign of the hydrogen atomic charges in the two molecules. As a general rule, low intensities can be expected for intermolecular modes with the exception of those which involve motions of hydrogens that appreciably alter the magnitude or direction of a subunit’s dipole moment.

Activation and Cleavage of H-R Bonds through Intermolecular H...H Bonding upon Reaction of Proton Donors HR with 18-Electron Transition Metal Hydrides

G. Orlova et al. — Thu, 08 Sep 2011 14:52:16 PDT

The H−R cleavage upon reaction MH + HR → MH···HR → M(η²-H₂)R, where MH represents 18-e trans-dihydrides Ru(H)(H)(PH₂CH₂PH₂)₂ (1), Ru(H)(H)(PH₃)₄ (2), Ru(H)(H)(NH₃)₄ (3); HR are HX (X = F, Cl) and HOR (R = H, CH₃) is studied using the DFT B3PW91/LANL2DZ level of theoretical calculations. The H−R bond splits upon interaction of the HR with 1 and 3 which possess a hydride H of high proton attracting power and significantly electropositive H of PH₂ and NH₃ groups. The basicity of the transition metal plays only a minor role in H−R splitting. The H−R cleavage proceeds via transfer of the H atom from R to hydride H in Ru−H···H−R···H−P(N), as an exothermic process without barrier or H···H intermediate. The less acidic HOR yields a multi-H-bonded intermediate Ru−H···H−O···(H−P(N))₂, where the H−O bond cleaves with a low barrier. Such an energetically facile mechanism of H−R splitting was not found for 2, where H of PH₃ is too inert to interact with R and a multi-H-bonded complex is not formed. The computed relative energies and barriers are in agreement with available experimental data.

Ab Initio Studies of the Structure, Energetics, and Vibrational Spectra of Hydrogen-Bonded Systems

Steve Scheiner — Thu, 08 Sep 2011 14:52:06 PDT

Existence and Characterization of HOO-HOOOH Radical-Molecule Complexes: A Computational Study

M. Solimannejad et al. — Thu, 08 Sep 2011 14:51:57 PDT

Correlated Proton Transfer Potentials. (HO-H-OH)- and (H2O-H-OH2)+

Z. Latajka et al. — Thu, 08 Sep 2011 14:51:48 PDT

Comparison of Proton Transfers between Carbonyl and Hydroxyl Groups

Steve Scheiner — Thu, 08 Sep 2011 14:51:39 PDT

Theoretical Investigation of the Dihydrogen Bond Linking MH2 with HCCRgF (M=Zn,Cd; Rg=Ar,Kr)

M. Solimannejad et al. — Thu, 08 Sep 2011 14:51:30 PDT

Ab Initio Investigations of the Hydrolysis of the Carbamate Bond

M. Remko et al. — Thu, 08 Sep 2011 14:51:20 PDT

Boron-Nitrogen (BN) Substitution Patterns in C/BN Hybrid Fullerenes: C60-2x(BN)x (x=1-7)

J. Pattanayak et al. — Thu, 08 Sep 2011 14:51:11 PDT

Theoretical study of H2O–HF and H2O–HCl: Comparison with experiment

M. M. Szczesniak et al. — Thu, 08 Sep 2011 14:51:01 PDT

The H bonds in H₂O–HF and H₂O–HCl are studied and compared using ab initio molecular orbital methods and the results compared to experimental data. Basis sets used are: (i) triple valence 6‐311G∗∗ and (ii) double ζ with two sets of polarization functions. Electron correlation, included via second‐ and third‐order Møller–Plesset perturbation theory, is found to have profound effects on both systems, particularly H₂O–HCl. Both H bonds are strengthened substantially with a concomitant reduction in length. H‐bond energies and geometries calculated at correlated levels are in excellent accord with available experimental information. In both systems, all levels of theory indicate the equilibrium geometry contains a pyramidal arrangement about the oxygen atom. However, the difference in energy between this structure and a C_2v planar arrangement is found to be small enough that consideration of probability amplitudes in the ground vibrational level leads to nearly equal likelihood of observing either geometry. Agreement between experimental vibrational frequencies in H₂O–HF and those calculated at correlated levels and involving quadratic, cubic, and quartic force constants is quite good. An explanation is offered for the increase in HX bond length which occurs at SCF and correlated levels upon H‐bond formation based upon nearly linear relationships between this length on one hand and subunit dipole moment and polarizability on the other. The dispersion energy is found to be a very sensitive, almost exactly linear function of the increase of H–X bond length. This energy contributes substantially to the weakening of the HX bond upon complexation.