Molecular structure and infrared spectrum of protonated nitrous oxide

Ab initio molecular-orbital theory is applied to the protonated N₂O isomers HNNO⁺ and NNOH⁺ in the light of the recent high-resolution spectroscopic detection of this molecular ion by Amano. Only the oxygen-protonated species is consistent with the experimental observations of Amano.     

Electronic structure and vibrational spectrum of acetylthiocarbamide

An approximate quantum chemical optimization of the geometric parameters of the acetylthiocarbamide molecule CH₃CONHCSNH₂ was carried out using the MNDO/H approximation. Bond lengths, bond angles, enthalpy of formation, total energy, ionization potential, and dipole moment were estimated, and the effective charges on the atoms and the bond orders were calculated. An analysis of the normal vibrations of the acetylthiocarbamide molecule and its deuteroanalog CH₃CONDCSND₂ was carried out. The force fields have been estimated. The frequencies, potential energy distribution among the vibrational coordinates, and the frequencies for the partially and completely deuterated acetylthiocarbamide molecules have been calculated.A. A. Baikov Institute of Metallurgy, Russian Academy of Sciences. A. A. Sechenov Moscow Academy of Medicine. N. S. Kurnakov Institute of General and Inorganic Chemistry. Translated from Zhurnal Strukturnoi Khimii, Vol. 34, No. 2, pp. 58–65, March–April, 1993.     

Electronic spectrum of TaO and its hyperfine structure

The B (2)Phi(5/2)-X(1) (2)Delta(3/2)(0,0) band at 778 nm and the C (2)Delta(3/2)-X(1) (2)Delta(3/2)(0,0) band at 737 nm of tantalum oxide (TaO) were recorded by laser excitation spectroscopy using a hollow cathode sputtering source to generate the molecules. The hyperfine structure arising from the (181)Ta (I=72) nucleus was measured at sub-Doppler resolution using the technique of intermodulated fluorescence spectroscopy. The hyperfine structure was assigned and fitted in order to derive accurate values for the magnetic dipole and electric quadrupole interactions. The magnetic hyperfine constant for the ground electronic state was also calculated using the density functional theory as h(3/2)=625 MHz, in good agreement with the experimental value of 647+/-10 MHz. This result suggests that the X (2)Delta ground state of TaO is well described by a pure deltasigma(2) electronic configuration, where the unpaired electron is located in a Ta 5ddelta orbital.     

Electronic structure and photoelectron spectrum Assignment of allene

In this paper a series of ab initio SCF and configuration calculations were reported forthe ground state and excited states X~2E, A~2E,~2B_2 and ~2A_1 of allene.For ground state X~2E Jahn-Teller distorsion was discussed and a twisted angle of 50° and a torsional barriers of 0.21—0.51 eVwere derived.Based on calculated results,the experimental photoelectron spectrum of allene has beenassigned.     

Electronic spectra of protonated ferrocenes

The electronic absorption spectra of two protonated ferrocenes have been recorded and the results require a reaccessment of the identity of the species present in weak acid.     

Electronic structure and N KVV Auger spectrum of the ammonium radical

A procedure for calculating multiplet-splitting energies in systems with an unpaired electron has been presented. The electronic structure and N KVV Auger spectrum of the ammonium radical have been calculated by the discrete-variation-X method. The results have been compared with experimental data.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 1, pp. 10–18, January–February, 1985.     

The structure of protonated disilene

The open and cyclic forms of Si₂ H₅⁺ have been calculated at the SCF level and with inclusion of electron correlation energy. Our final results indicate that both structures are about equally stable. The proton affinity of disilene is calculated as 207 kcal/mol.     

Electronic structures and configurational transformations of some protonated compounds

The equilibrium geometries of HCHOH⁺, CH₃CHOH⁺, (CH₃)₂COH⁺, HCO₂H⁺₂, and CH₃CO₂H⁺₂ were determined by means of the semiempirical INDO—SCF method. The total-energy difference between the protonated isomers was found to be reasonable as compared with that obtained from the NMR observations. The computed barriers of the transformation between the isomers via the pure or distorted rotation or the bending motion were in good agreement with the experimental results.     

Electronic spectroscopy of cold, protonated tryptophan and tyrosine

We report here a new method to obtain electronic spectra of biomolecular ions that are produced in the gas phase by electrospray and cooled to approximately 10 K in a 22-pole ion trap, and we demonstrate this technique by applying it to protonated tryptophan and tyrosine. Cooling in the trap greatly simplifies the spectrum of protonated tyrosine, which exhibits a well-defined band origin and clearly resolved low frequency vibrational bands. In contrast, the spectrum of protonated tryptophan exhibits only broad features, even at low temperatures, suggesting that a fast nonradiative process broadens the individual vibronic features, even upon excitation at the electronic band origin. The method demonstrated here should be applicable to a wide variety of biological molecules.     

The electronic spectrum of protonated adenine: theory and experiment

In this work we present the results of a combined experimental and theoretical study concerned with the question how a proton changes the electronic spectrum and dynamics of adenine. In the experimental part, isolated adenine ions have been formed by electro-spray ionisation, stored, mass-selected and cooled in a Paul trap and dissociated by resonant photoexcitation with ns UV laser pulses. The S(0)-S1 spectrum of protonated adenine recorded by fragment ion detection lies in a similar energy range as the first pipi* transition of neutral 9H-adenine. It shows a flat onset with a broad substructure, indicating a large S(0)-S1 geometry shift and an ultra-short lifetime. In the theoretical part, relative energies of the ground and the excited states of the most important tautomers have been calculated by means of a combined density functional theory and multi-reference configuration interaction approach. Protonation at the nitrogen in position 1 of the neutral 9H-adenine tautomer yields the most stable protonated adenine species, 1H-9H-A+. The 3H-7H-A+ and the 3H-9H-A+ tautomers, formed by protonation of 7H- and 9H-adenine in 3-position, are higher in energy by 162 cm(-1) and 688 cm(-1), respectively. Other tautomers lie at considerably higher energies. Calculated vertical absorption spectra are reported for all investigated tautomers whereas geometry optimisations of excited states have been carried out only for the most interesting ones. The S1 state energies and geometries are found to depend on the protonation site. The theoretical data match best with the experimental onset of the spectrum for the 1H-9H-A+ tautomer although we cannot definitely exclude contributions to the experimental spectrum from the 3H-7H-A+ tautomer at higher energies. The vertical S(0)--> S1 excitation energy is similar to the one in neutral 9H-adenine. As for the neutral adenine, we find a conical intersection of the S1 of protonated adenine with the ground state in an out-of-plane coordinate but at lower energies and accessible without barrier.     

The structure of protonated diamines and polyamines

The reduced mobilities in air, at 200C, of six isomeric C₇H₁₈N₂ protonated diamines, two triamines (caldine and spermidine), and two tetramines (thermine and spermine) were measured by ion mobility spectrometric (IMS) techniques. The results indicated that all these polyamines undergo proton-induced cyclization, with the proton forming a bridge between two amino groups. It appears as if the favored configuration of the protonated polyamines involves a six- or seven-membered ring rather than a bridge between the terminal amino groups. It is believed that in the tetramines the cyclic structure is formed between the two central, more basic, secondary amine sites.     

IR spectrum and structure of protonated ethanol dimer: implications for the mobility of excess protons in solution

This Communication reports IR spectra and density functional calculations for the isolated protonated ethanol dimer and its N2-microsolvated complexes, (EtOH)2H+-(N2)n (n = 0-2) to investigate the degree of delocalization of the excess proton in this fundamental building block of an alcohol proton wire. The first spectroscopic characterization of isolated and microsolvated (EtOH)2H+ suggests that the excess proton is (nearly) equally shared between both EtOH units under symmetric solvation conditions (Zundel-type ion, n = 0 and 2), whereas it is largely localized on a single EtOH molecule for asymmetric solvation (Eigen-type ion, n = 1).     

Electronic spectra and structures of protonated pyrazine and quinoxaline N-oxides

The electronic spectra of neutral molecules and monocations of N-oxides of 2- and 3-substituted pyrazines and quinoxalines were measured, and their ionization constants in water were determined. The position of the protonation center in the investigated compounds was established.     

Electronic structure and vertical excitation spectrum of methylene amidogen CH2N

Ab initio electronic structure calculations are reported for five electronic states of the methylene amidogen radical. Structure parameters for the ground electronic state are predicted by RHF and D -MBPT (4) calculations. Vertical excitation energies were determined using four different theoretical chemical models: complete active space (CAS ) MCSCF , CAS /MCSCF plus singles and doubles Cl, fourth-order many-body perturbation theory SDQ -MBPT (4), and coupled-cluster theory.     

On the structure of protonated methane

Results of a Fourier transform-ion cyclotron resonance study are reported concerning the reactivity of protonated perdeuteromethane and deuteronated methane, generated under varying pressure conditions in an external chemical ionization ion source, toward ammonia. The competition between proton and deuteron transfer from both protonated perdeuteromethane and deuteronated methane to ammonia exhibits chemically distinguishable hydrogens. The chemical behavior of protonated methane appears to be compatible with the theoretically predicted stable structure with C_S symmetry, involving a three-center two-electron bond associating two hydrogens and the carbon atom. Interconversion of this structure due to exchange between one of these hydrogens and one of the three remaining hydrogens appears to be a fast process that is induced by interactions with the chemical ionization gas.     

The mobility and ion structure of protonated aminoazoles

The reduced mobility of protonated pyrazole derivatives was measured by ion mobility spectrometry (IMS) in air, nitrogen, and carbon dioxide, at temperatures between 150 and 250°C. It was found that the mobility of protonated 5-amino-1-phenylpyrazole was higher than that of its 3- and 4-isomers. This was attributed to the fact that in the 5-isomer the preferred site of protonation is on the endocyclic nitrogen, which leads to delocalization of the ionic charge, and thus to a diminished interaction with the drift gas molecules. On the other hand, protonated 5-amino-1-methylpyrazole has a slightly lower mobility than its isomers, which is indicative of a different protonation mechanism.     

Electronic structure study of the high-pressure vibrational spectrum of FeS2 pyrite

Plane-wave density functional calculations are used to investigate the pressure dependence of the geometry and Gamma-point phonons of FeS(2) pyrite up to 150 GPa. The linear response method is employed to calculate the vibrational properties. Raman-active modes are in excellent agreement with the experimental data available up to 50 GPa,(1) and we predict the evolution with pressure of the IR-active modes for which no high-pressure spectroscopic data have been reported so far. Over the wide pressure range investigated here, all vibrational frequencies depend nonlinearly on pressure; their pressure dependence is quantified by determining the full set of mode Grüneisen parameters and their pressure derivatives.