MP2 6-31G Research Articles

The conversion of 3- and 4-hydroxybenzonitriles ( m- and p-cyanophenols) into oxyanions, followed by IR spectra, ab initio and density functional calculations

The conversion of 3- and 4-hydroxybenzonitriles ( m- and p-cyanophenols) into oxyanions has been followed by IR spectra, ab initio HF, MP2 6-31G(d) and density functional BLYP, B3LYP 6-31G(d) force field calculations. In a general agreement between theory and experiment, the conversion causes a 34 cm −1 frequency decrease in the cyano stretching band, 3.2-fold increase in its integrated intensity and other essential spectral changes in the para-isomer; for the meta-one the corresponding values are 13 cm −1 and 2.3-fold, respectively. According to the calculations, both molecule and anion of 4-hydroxybenzonitrile are more stable (due to the polar resonance) than the corresponding meta-substituted species. The strongest structural changes, caused by deprotonation, are the 0.105 Å shortenings of the Ph–O bonds, 0.054 Å lengthenings (mean values) of the adjacent CC bonds and essential bond angle changes in the phenylene rings near the oxyanionic centers. In the case of para-isomer these changes are related to the formation of a quasi- para-quinonoidal structure of the phenylene ring in the oxyanion. According to the electronic density analysis, a bit less (Mulliken) or a bit more (natural bond orbital) than halves of the anionic charges remain localized at the oxyanionic centers. Hydrogen bonds have also been discussed.

Neutralization of methyl cation via chemical reactions in low-energy ion-surface collisions with fluorocarbon and hydrocarbon self-assembled monolayer films

Low-energy ion-surface collisions of methyl cation at hydrocarbon and fluorocarbon self-assembled monolayer (SAM) surfaces produce extensive neutralization of CH 3 +. These experimental observations are reported together with the results obtained for ion-surface collisions with the molecular ions of benzene, styrene, 3-fluorobenzonitrile, 1,3,5-triazine, and ammonia on the same surfaces. For comparison, low-energy gas-phase collisions of CD 3 + and 3-fluorobenzonitrile molecular ions with neutral n-butane reagent gas were conducted in a triple quadrupole (QQQ) instrument. Relevant MP2 6-31G*//MP2 6-31G* ab initio and thermochemical calculations provide further insight in the neutralization mechanisms of methyl cation. The data suggest that neutralization of methyl cation with hydrocarbon and fluorocarbon SAMs occurs by concerted chemical reactions, i.e., that neutralization of the projectile occurs not only by a direct electron transfer from the surface but also by formation of a neutral molecule. The calculations indicate that the following products can be formed by exothermic processes and without appreciable activation energy: CH 4 (formal hydride ion addition) and C 2H 6 (formal methyl anion addition) from a hydrocarbon surface and CH 3F (formal fluoride addition) from a fluorocarbon surface. The results also demonstrate that, in some cases, simple thermochemical calculations cannot be used to predict the energy profiles because relatively large activation energies can be associated with exothermic reactions, as was found for the formation of CH 3CF 3 (formal addition of trifluoromethyl anion).

Infrared and Raman spectra, vibrational assignment, and ab initio calculations of fluoromalononitrile

The infrared spectra (3500-80 cm −1) of gaseous and solid and the Raman spectra (3500-30 cm −1) of liquid with quantitative depolarization values and solid fluoromalononitrile, FHC(CN) 2, have been recorded. These spectra have been interpreted in detail and a complete vibrational assignment is proposed based on infrared band contours, depolarization values, and relative infrared and Raman intensities. The assignment is supported by a normal coordinate calculation based upon force constants obtained from MP2 6-31G ∗ ab initio calculations. Also, from these calculations the predicted infrared and Raman spectra have been obtained. The CH bond distance is predicted to be 1.096 Å from the isolated carbon-hydrogen stretching frequency. Optimized geometric parameters have been obtained from RHF 6-31G ∗ , MP2 6-31G ∗ and MP2 6-311 + + G ∗∗ ab initio calculations. These results are discussed and compared with those obtained for some similar molecules.

Determination of the excited state geometry of NSO-containing compounds by means of the pre resonance Raman effect. Study on bis(sulfinylamino)sulfane, S(NSO) 2

The study of the pre resonance Raman effect of bis(sulfinylamino)sulfane, S(NSO) 2, was performed in order to obtain information about the electronic excited state geometry of the molecule. The structure and conformation of S(NSO) 2 were determined by ab initio calculations ( HF 3-21G , HF 6-31G , HF 3-21G ∗ with an additional set of d-functions at the nitrogens, α d = 0.8, HF 6-21G ∗ , HF 6-31G ∗ and MP2 6-31G ∗ ). The molecule possesses C 2 v symmetry with both SO bonds in syn ( cis) position with respect to the SN bonds. The vibrational spectra were also investigated. A complete assignment of the observed bands is proposed and correlated with results arising from theoretical calculations. Subsequently, a normal coordinate analysis is performed with the experimental data. Excited state geometries of S(NSO) 2 were determined from experimental data using an expression relating the Raman intensities to the frequencies of the normal modes and the dimensionless displacements. It takes also into account both the difference in the electronic energy between the minimum of the fundamental state potential surface and the minimum of the excited state potential surface and the energy of the incident radiation. The excitation profile was calculated for the six incident radiations. Both, transferability of the RNSO configuration and ability of the pre resonance Raman effect studies to give results related to the molecular geometries were tested again.

Infrared vibrational intensities and polar tensors of the carbonyl and thiocarbonyl halides

The polar tensors of F 2CO, Cl 2CO, F 2CS and Cl 2CS have been determined from experimental infrared intensities. The sign ambiguities in the dipole moment derivatives were resolved by comparing alternative polar tensor solutions with the results of MP2 6-31G ∗ molecular orbital calculations using bidimensional principal component projections of the polar tensor spaces. The signs chosen for the dipole moment derivatives are the same as those chosen in an earlier work where CNDO estimates were used for comparison. Signs for derivatives related to very small intensities (< 0.5 km mol −1) were left undetermined. As already noted before the polar tensor for an atom of one of these carbonyl or thiocarbonyl halides can be estimated from the atomic polar tensors of the other three molecules. This is also true for the mean dipole moment derivatives and infrared intensity sums of these molecules. These simple relations can be explained if one assumes no saturation effect on the capacity of the carbon atom to donate electron density to the terminal atoms. Furthermore these simple relations appear to be extendable to the 1s core electron energies of these molecules if the mean dipole moment derivative values are used as atomic charges in the simple potential model.

Examination of Sputtered Ion Mechanisms Leading to the Formation of C7H7+ during Surface Induced Dissociation (SID) Tandem Mass Spectrometry (MS/MS) of Benzene Molecular Cations

Recent work reported in the literature has shown new evidence of a sputtered ion mechanism for associative ion surface reactions occurring during surface induced dissociation (SID) tandem mass spectrometry (MS/MS). In the context of a sputtered ion mechanism, we have studied selected routes to the formation of C7H7+ during the low-energy collision (ca. 30 eV) of benzene molecular ions with surfaces covered with a hydrocarbon overlayer. A key approach utilized in this work is the use of gas phase ion molecule chemistry to model reactive ion surface collisions. Benzene, 2H6-labeled benzene, and 13C6-labeled benzene have been examined by SID and collision activated dissociation (CAD). The CAD experiments have been used to investigate the fragmentation of hydrocarbon adducts of benzene and labeled benzene formed by methane and isobutane chemical ionization (CI) as models of those ions which may be formed via a sputtered ion mechanism during SID. In addition, the thermodynamics of many of the possible sputtered ion routes to the formation of C7H7+ have been compared using experimental heats of formation as well as total energies and zero-point vibrational energies (ZPVE) obtained by ab initio (MP2 6-31G*//HF 6-31G*) calculations. While there are several seemingly viable sputtered ion routes to the formation of C7H7+ during SID of benzene molecular ions, the combination of thermodynamic considerations and experimental results suggest that a likely sputtered ion route involves the reaction of neutralized benzene ions with C3H5+ ions (sputtered from the hydrocarbon overlayer) followed by the loss of ethene.

Density Functional Theory Study of Molecular Structures and Vibrational Spectra of 3,4- and 2,3-Pyridyne

Density functional theory and ab initio MP2 6-31G* calculations were carried out to investigate the structures and vibrational spectra of 3,4- and 2,3-pyridyne. It is found that the structure of 3,4-pyridyne is consistent with a formal CC triple bond moiety, but the structure of 2,3-pyridyne is more properly described as having a unit. On the basis of the calculated results, detailed assignments of the observed IR bands of 3,4-pyridyne are proposed. The calculations predict the most prominent IR feature of 2,3-pyridyne is a very strong band around 1826 cm-1, suggesting the search for direct experimental evidence of 2,3-pyridyne should pay attention to this spectral region.

An ab initio study of the molecular complexes formed between H 2S and the acid sites of zeolites

Molecular complexes formed by the interaction of hydrogen sulphide H 2S with silanol H 3Si(OH), two model Brønsted acid sites of zeolite clusters H 2Si (OH) AlH 3 and H 3Si (OH) Al(OH) 2SiH 3 and a sodium zeolite cluster H 3Si (ONa) Al (OH) 2SiH 3 have been studied by ab initio methods. Both Hartree Fock (HF) and post-Hartree-Fock (MP2) levels of theory were used. The geometries of the isolated molecules and the corresponding complexes were optimized at the HF/6-31G level of theory, assuming C s symmetry for the complexes. Corresponding geometry optimizations at the MP2 6-31G” level of theory were also carried out for the H 2S silanol and the H 2S-H 3Si(OH)AlH 3 complexes. Comparisons with similar water zeolite complexes are made. The HF harmonic vibrational frequencies of the OH and SH stretching modes of these complexes are compared with experimental infrared measurements.

Conformational stability of trans-1-fluoro-2-butene

Variable temperature infrared studies of trans-1-fluoro-2-butene, trans-CH3HCCHCH2F, have been carried out with the sample dissolved in liquid xenon. These spectral data have been interpreted on the basis that the molecule exists in the fluid phases as a mixture of the synclinal (fluorine atom oriented cis to the double bond) and anticlinal (gauche) conformations and the synclinal conformer is more stable by 130 ± 40 cm−1 (372 ± 114 cal/mol). Ab initio calculations have been carried out utilizing the RHF6–31G∗∗The N.F.W.O. (Belgium) is acknowledged for a grant in support of W.A.H. to carry out this research. and MP26–31G∗ basis sets which predicted the cis rotamer more stable than the gauche form which is consistent with the experimental results.

Is there a hydride transfer between N2OH+ and saturated hydrocarbons?

Various reaction mechanisms for the reaction between N 2 OH + and CH 4 have been explored theoretically at the MP2 6-31G ∗∗ level. Also computed were the structure and thermodynamic state functions for several neutral molecules, notably N 2 O, CH 4 and C 2 H 6 and ions (CH 5 + , C 2 H 5 + , C 2 H 7 + , N 2 OH + ). These results were combined with a Fourier transform ion cyclotron resonance study of the reactions of N 2 OH + with CH 4 or C 2 H 6 . Both studies suggest that the title hydride transfer, while thermodynamically favorable, is not Kinetically preferred under thermal conditions in the gas phase.

Photoelectron spectrum and reactivity of silylalkyl sulphides. Stabilization of radical cations by the β-silyl effect

The effect of a β-silyl group on sulphur lone pair ionization energies has been re-investigated by use of ab initio and semi-empirical MNDO quantum-chemical calculations. The effect of the interaction between n s and σ SiC★ orbitals and that of electron reorganization have been shown to be unimportant in the case of silyl-methyl-sulphides. To interpret the observed features of the ionization energies and reactivity, the electron release by the β-silyl group and hyperconjugative interactions have been considered. Stabilization brought about by β-substitution for the ground state of the radical cation is ca. 10 kcal mol −1 for alkyl (10.11 kcal mol −1) and silyl (9.48 kcal mol −1) substituents at the MP2 6-31G★★//HF 6-31G★★ level of theory. The effects of the substituents are smaller on the neutral ground state of the molecules (6.80 and 0.50 kcal mol −1 for methyl and silyl groups, respectively) at the same level of theory. The observed low ionization energy of the β-silyl-substituted sulphides must be attributed to differences in the stabilization of the neutral ground states of the silyl- and alkyl-substituted derivatives rather than to the stabilization of the radical cationic ground state in the case of compounds of the type R 3SiCH 2SH.

Ab initio studies of bonding trends: Part 8. The 26-electron AB-CD n and the 30-electron ABCD n systems

The isoelectronic series of NCCN and OCCO 2− are screened for eventual new species at the HF or MP2 6-31G ∗ level. The new cyanogen analogs include the carbide C 4 2−, the phosphacyanide acetylide CCCP −, and the cations NCCO +, NCNN + and NNNN 2+. The new 30-electron linear species include NCCN 4−, OCBO 3−, FNCF + and FNNF 2+. For the known ONCO −, a ring structure lies above the linear one, while the experimentally unknown N 4 2− prefers the D 4h structure. The O 4 2+ ( D 4h) ring is stable at the HF level but unstable at the MP2(6-31G ∗) level.

Ab initio studies of bonding trends: Part 9. The dicyanamide-carbon suboxide-dicyanoether-cyanogen azide isoelectronic series ABCDE 1

The isoelectronic analogs of the title molecules are screened for possible unknown species using ab initio HF or MP2 6-31G ∗ calculations. Some candidates are the bent cations OCNCO + and OCPCO +, the linear anions OBCBO 2− and OBNBO −, and the bent NBNBN 3− and NBOBN 2−. The known NC-NNN is predicted to have a CN 4( D 2d) di(diazirine) isomer, only 2.7 eV above the cyanogen azide at HF level. The analogous BN 4 − is also found to have a local minimum. The predicted neutral species include the linear OCNBO and ONNBN and the bent OCPCN, rather resembling the experimentally known NCPCN − anion.

Theoretical study of some BXHn compounds where X stands for B, C, N, O or F

In this work, we investigate theoretically the energetics of some of the simplest boron-boron boron-carbon, boron-nitrogen, boron-oxygen and boron-fluorine compounds. Optimized structures have been carried out at the HF (RHF and UHF) and MP2 6-31G** level . Our attention has been focused on the heats of formation. They have been obtained at the MP4 level of the theory, using basis sets including polarized (d, f) and diffuse (+) functions. Bond dissociation energies have also been obtained. They arc closely related to the electronic structures. Singlet BB, HBBH, BCH and HBC compounds exibit particularly low bond dissociation energies, but their fundamental states arc known to be triplet instead of singlet state.