CS Stretch Research Articles

Photodissociation Spectra of OCS+ via B2Σ+↙X2Π Transitions

In the wavelength range of 231–275 nm, we have studied the mass-resolved dissociation spectra of OCS+ via B2Σ+↙X2Π3/2(000) and B2Σ+↙X2Π1/2(000, 001) transitions by preparing OCS+ ions in the well-defined spin-orbit states. The spectroscopic constants of ?1(CS stretch)=828.9 (810.4) cm−1, ?2 (bend)=491.3 cm−1 and ?3(CO stretch)=1887.2 cm−1 for OCS+(B2Σ+) are deduced. The observed dependence of the ?2(bend) mode excitation of B2Σ+ on the spin-orbit splitting of X2Π(Ω=1/2, 3/2) in the B2Σ+↙X2Π transition can be attributed to the K coupling between the (000)2Π1/2 and (010)2Σ1/2+ vibronic levels of X2Π state, which makes the B2Σ+(010)↙X2Π1/2(000) transition possible.

Palladium(II) and Platinum(II) Complexes of N-Phenyl- and N-Ethyl-N′-pyrimidin-2-ylthiourea

Palladium(II) and platinum(II) complexes of N-ethyl-N′-pyrimidin-2-ylthiourea(HL1) and N-phenyl-N′-pyrimidin-2-ylthiourea (HL2) have been prepared, and the complexes [M(HL)Cl2], [Pt(L)2], [Pd(HL1)2]Cl2, and [Pd(L2)2] (where M = PdII or PtII) were characterized. The spectroscopic data are consistent with coordination of thioureas as neutral or monoanionic ligands to PdII and PtII through S and a pyrimidine-N. The IR spectra show shifts of CS and pyrimidine ring stretch bands to lower and higher frequencies, respectively. The 1H NMR spectra differentiate between H(4′) and H(6′) resonances and indicate downfield shifts for all protons of pyrimidine [H(4′), H(5′), and H(6′)], two resonances for two N‒H protons for complexes containing the neutral ligand (HL), and only one N‒H proton chemical shift for complexes containing the monoanion (L). 13C NMR chemical shifts of pyrimidine carbons are correlated with the type of bonding between PdII or PtII and pyrimidine-N. The magnetic susceptibilities suggest a diamagnetic planar structure for all complexes. Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file.

ChemInform Abstract: Vibrational Studies of the Disulfide Group in Proteins. Part 3. A Simplified ab initio Force Field for Diethyl Disulfide and SS and CS Stretch Frequency-Conformation Correlations for Diisobutyl Disulfide.

ChemInform Abstract: Vibrational Studies of the Disulfide Group in Proteins. Part 3. A Simplified ab initio Force Field for Diethyl Disulfide and SS and CS Stretch Frequency-Conformation Correlations for Diisobutyl Disulfide.

Rotationally Resolved Ground State Vibrational Levels of HC2S Studied by Two-Color Resonant Four-Wave Mixing

A high-resolution study of the X(2)Pi(3/2) ground state rovibronic energy levels of jet-cooled HC(2)S radical using the double-resonance two-color four-wave mixing technique (TC-RFWM) is reported. The rotational structure of the bands is observed by excitation of selected R-branch lines in the origin band of the A(2)Pi(3/2)-X(2)Pi(3/2) electronic system. The second laser frequency is scanned to transfer the population from the rotational level(s) of the upper state to selected vibronic levels of the ground state. Fourteen rotationally resolved vibrational bands have been recorded for energies up to 1800 cm(-1) above the v'' = 0 X(2)Pi(3/2) electronic ground state. Effective rotational constants and origins are determined for levels that involve fundamental and overtone combinations of the nu(3) (CS stretch), nu(4) (CCH bend), and nu(5) (CCS bend) vibrations. This illustrates the power and advantages of the TC-RFWM approach for the study of the ground state manifold of reactive intermediates produced in low concentrations with high resolution, good signal-to-noise and wide dynamic range.

Resonance structure in low-energy electron scattering from OCS

Energy-dependent absolute angle-differential cross sections (θ = 135°) for elastic and vibrationally inelastic electron scattering from OCS molecules have been investigated at high resolution (13 meV). The elastic cross section, reported over the range E= 0.06–20 eV, exhibits a deep Ramsauer–Townsend minimum near 0.55 eV and the π*(1.2 eV) and σ*(3.8 eV) resonances known from earlier experimental and theoretical work. The excitation functions for vibrational excitation (VE) of the fundamental CS stretch (1 0 0), the CO stretch (0 0 1) as well as for the fundamental and harmonics of the bending (0 n 0) mode are reported from threshold up to E = 8 eV. VE is mediated—in part in a mode-selective way—by threshold peaks and the higher lying resonances. The selectivity in the threshold region can be rationalized in terms of symmetry arguments where the 2Σ virtual state excites preferentially the σ vibrational states with even quanta of bending vibration over the π vibrational states with odd quanta of bending vibration. The σ*CS resonance is essentially inactive in VE of the CO stretch and the bending mode.

Laser induced fluorescence spectroscopy of the à 3Πi←X̃ 3Σ− transition of the CCS radical

The à 3Πi←X̃ 3Σ− electronic transition of a carbon-chain molecule CCS has been observed for the first time by laser induced fluorescence spectroscopy. The species was generated with a pulsed discharge of 0.5% C2H2 and 0.5% CS2 diluted in Ar via a pulsed supersonic jet. A number of bands in the 690–675 nm and 600–615 nm regions (14 500–14 800, 16 200–16 600 cm−1) were observed and assigned to those of CCS using ground state combination differences. Three of the bands in each region were found to belong to the three à 3Πi spin-orbit components of the à 3Πi←X̃ 3Σ− transition (Tv=14 662.777(4) and 16 423 cm−1), and an effective set of rotational constants for the upper states were obtained. The higher energy band was heavily perturbed, preventing the determination of effective constants for this band. These bands have been tentatively assigned as two successive bands in the à 3Πi(n,0,0) progression with a resultant effective value of 1760 cm−1 for the à 3Πi ν1 vibrational fundamental. Dispersed fluorescence spectra from the à 3Πi←X̃ 3Σ− band of CCS have also been observed, yielding definitive experimental information on the vibrational structure of the X̃ 3Σ− ground state for the first time. The three harmonic frequencies were determined to be 1708.2(40) cm−1, 862.5(19) cm−1, and 269.3(13) cm−1 for ω1 (CC stretch), ω2 (CS stretch), and ω3 (CCS bend), respectively.

Vibrational analyses of the tetrathiosquarate ion based on ab initio molecular orbital and density functional calculations: Effect of the Jahn-Teller distortion in the excited electronic state on Raman intensities

The vibrational force field of the tetrathiosquarate ion (C 4S 4 2− is analyzed on the basis of the observed infrared and Raman spectra and ab initio molecular orbital (MO) and density functional (DF) calculations. Definite assignments are made for the main infrared and Raman bands. The intensity pattern in the observed Raman spectrum is reasonably well reproduced by the ab initio MO calculations at the third-order Møller-Plesset perturbation level and by the DF calculations. Electron correlation is seen to have a strong effect on the Raman intensities of bands which have a significant contribution of the CS stretch. The relationship between the Raman intensities and the Jahn-Teller distortion in the excited electronic state (the intermediate state in the Raman process) is examined by calculating the optimized structures at the stationary points (the rectangular, rhombic and ‘deformed rhombic’ forms) on the potential energy surface of that state. It is shown that these optimized structures are consistent with the character of the non-totally symmetric modes, which give rise to strong Raman bands. It is suggested that the appearance of a band arising from the first overtone of an out-of-plane mode in the observed pre-resonance Raman spectrum originates from the character of the rhombic form as a saddle point for deformation in an out-of-plane direction.

The spectroscopy of thioformaldehyde. MR-CI studies on the triplet states of H2CS

Abstract Using multireference CI methods, the CS stretching (C2v) and out-of-plane (Cs) potentials were calculated for triplet states of H2CS. From the CS stretch potentials the adiabatic excitation energies, CS equilibrium distances, vibrational frequencies and Franck-Condon (FC) factors were evaluated. Both the 3 ( σ, π ∗ ) and 3 ( π, π ∗ ) ← X systems are predicted to be FC active, leading to long progressions. The 3 ( σ, π ∗ ) state is found in the same energy region as S2, and may therefore play a role in the unusual photo-chemistry and photo-physics assocaited with S2 ← S0. It is shown that 3 ( π, π ∗ ) is nonplanar, with a stabilization of ∼ 0.05 eV over the planar configuration, having RCS = 3.48 a0 and an out-of-plane angle of ∼ 28°. Also, 3 ( σ, π ∗ ) is predicted to be nonplanar, with RCS = 3.55 a0, θ ∼ 56° and a stabilization energy of ∼ 0.17 eV. The 3 ( π, π ∗ ) ← X system, while predicted to be weak, lies in an energy region free from other states. Electron impact spectroscopy offers the best chance of observing this system. Vertical excitation energies and dipole moments will also be given.

Vibrational mode and collision energy effects on a highly constrained reaction: OCS+(ν)+OCS→CS+2+CO2 and S+2+2 CO

We report the effects of collision energy and OCS+ vibrational state (ν1, ν2, and ν3) on the reaction of OCS+ with OCS. Production of CS+2+CO2 is exoergic and the cross section shows no evidence of an activation barrier. Nonetheless, the cross section is only ∼0.1% of the collision cross section, even at low collision energies where formation of an intermediate complex is facile. There appears to be a severe phase-space (steric) bottleneck for this rearrangement reaction. CS+2 production is weakly inhibited by collision energy, and enhanced by all three modes of OCS+ reactant vibrational excitation. Production of S+2 is endoergic and is enhanced by collision energy and by ν2 (bend) and ν3 (CS stretch) excitation. Excitation of ν1 (CO stretch) does not enhance this channel, even though it is the highest energy mode. At high collision energies, S+2 production becomes relatively efficient, suggesting that the reaction mechanism for this channel is direct with no significant bottleneck.

The spectroscopy of H2CS. 1. CS stretch potential curves for singlet states of planar H2CS, obtained by ab initio multireference CI methods

For planar H2CS, (C2ν), the CS stretch potential curves were obtained for the four to six lowest singlet states of each symmetry species by using multireference CI methods. Included were the (n, 4s), (n, 4p), (n, 3d), (π, 4s), and (π, 4p) Rydberg as well as the (n, π*), (π, π*), (σ, π*), (n, σ*), (n0, π*2), and (nπ, π*2) valence states. Vertical and adiabatic excitation energies, equilibrium CS distances, vibrational frequencies for the CS stretching mode, dipole moments, oscillator strengths, and Franck–Condon factors were evaluated and found to be in good agreement with known experimental data. The role of the 1(π, π*) state that diabatically crosses all 1A1 states, including the n2 ground-state configuration, causing many interactions with other states, has been given special attention. The following reassignments and predictions are of interest. (i) A switch of Ẽ and [Formula: see text], with 1A1(n, 4py) corresponding to the Ẽ bands and 1B2(n, 4pz) corresponding to the [Formula: see text] bands is suggested, based on the energetic ordering. (ii) Because of strong Franck–Condon factors, hot bands are suggested to play an important role in the analysis of the CS stretch progression of [Formula: see text]. (iii) The [Formula: see text] system, only studied in low resolution, is predicted to have high intensity and be perturbed due to the crossing of (π, π*) with (n, 4py) in the vertical region. The CS stretch bands should be observable. (iv) Observed combination modes in the [Formula: see text] system may be due to vibronic mixing of (π, π*) with (σ, π*).

Conformation dependence of the SH and CS stretch frequencies of the cysteine residue

SYNOPSIS In order to relate the observed SH and CS stretch frequencies of the cysteine residue in proteins more closely to its conformation, we have done normal mode calculations on a model for this structure, viz., (CCONH) (CNHCO)CHCH2SH. A range of x' and x2 were studied, combined with the $,$ of the a-helix, P-sheet, glutathione, and extended-helix conformations. The force field was a combination of a scaled ab initio force field of the -CH2SH group, obtained from ethanethiol and tested on 1-propanethiol and 3-thiol-Nmethylpropionamide, and our empirical force field for the peptide group. The results provide more detailed structure-spectra correlations than are possible from experimental studies of model compounds. 0 1992 John Wiley & Sons, Inc. I NTRODU CTl ON The cysteine residue side chain, -CH2SH, is an important one in proteins, both because of its inherent properties as well as its ability to react with similar groups to form a disulfide bridge, -CH2SSCH2-, between polypeptide chains. It is therefore important to develop detailed spectroscopic correlations for characterizing its conformation. It has long been known that the SH stretch (s) mode, v(SH), is generally found in the range of 2500-2600 cm-' , being weak in the ir and strong in the Raman spectrum. Since v(SH) is sensitive to the presence of SH groups and to their environment, particularly hydrogen bonding, it has been studied in various proteins, such as hemoglobin, 122 eye lens proteins, 3-5 0-lactoglobulin, and virus protein^.^,' Conformational information with respect to the CCs-S-H dihedral angle ( x2) has been sought through experimental studies of model alkanethiolsg- as well as normal mode calculations on such molecule~.'~~~~ Low-frequency torsional modes have also been studied14-16 in order to obtain information on rotational barriers. The v(CS) mode has been known to give rise to Raman bands in the 600-800-cm-' region. Confor

Vibrational studies of the disulfide group in proteins. VI. General correlations of SS and CS stretch frequencies with disulfide bridge geometry.

Normal mode calculations have been done on a range of disulfide bridge conformations in order to determine how the SS stretch and CS stretch frequencies depend on structure. In addition to varying the C alpha C beta SS and NC alpha C beta S dihedral angles, we have varied the phi, psi of the adjoining peptide groups since we have shown that these also influence the above frequencies. In order to obtain structural information from the observed frequencies, we have done a study of the conformational states found in 92 disulfide bridges in 25 known protein structures. This permits making a statistically based correlation between CS stretch frequencies and the possible contributing conformers.

Vibrational studies of the disulfied group in proteins part IV. SS and CS stretch frequencies of known peptide and protein disulfide bridges

We have used our previously derived ab initio disulfide and empirical polypeptide force field to calculate SS and CS stretch frequencies of disulfide bridges in a cyclic octapeptide and six proteins of known structure. Comparisons with Raman spectra show that the observed bands are very well reproduced by the normal mode calculations, thus validating the use of these force fields in studying detailed correlations between such spectra and disulfide bridge geometry.

Potential energy and dipole moment functions of the HCS radical

The three-dimensional MCSCF CI potential energy, electric dipole and electronic transition moment functions have been calculated for the X 2A′ and A 2A″ electronic states of the HCS radical. These states adiabatically correlate with the two components of a 2Π electronic state for linear configurations and their ro-vibronic spectra exhibit the Renner-Teller effect. Such spectra have been evaluated from variational ro-vibronic wavefunctions, which take into account both anharmonicity effects and rotation-vibration coupling. The nuclear and electronic angular momentum couplings have also been considered. Spectroscopic constants for the X 2A′ and A 2A″ states calculated by perturbation theory are given. The calculated equilibrium geometries are: R e CH: 1.083 Å (X), 1.063 Å (A), R e CS: 1.573 Å (X), 1.557 Å (A) and α e HCS: 131.8° (X), 180° (A), respectively. The electronic barrier to linearity in the electronic ground state is calculated to be 3063 ± 200 cm −1. The fundamental vibrational band origins and intensities (at 300 K) in the electronic ground state of HCS are predicted to be: 3104 cm −1/6 cm −2 atm −1 (CH stretch), 1165 cm −1/43cm −2 atm −1 (CS stretch) and 871 cm −1/54 cm −2 atm −1 (bend). The fundamental frequencies are expected to be accurate to within about 30 cm −1, the vibrational intensities to within about 10 to 20%. The electric dipole moment μ 0 (X) is calculated to be 0.85 ± 0.05 D. The K-reordering due to the electronic angular momentum coupling is discussed. Absolute line intensities (up to J″ = 9) have been calculated and are given for a few intense lines.

Vibrational studies of the disulfide group in proteins: Part III. A simplified ab initio force field for diethyl disulfide and SS and CS stretch frequency—conformation correlations for diisobutyl disulfide

We have obtained a simplified ab initio force field for diethyl disulfide, based on our previous scaled ab initio force field, which can be used in normal coordinate analysis of proteins containing the disulfide bridge. A normal coordinate analysis has been performed for diisobutyl disulfide in all possible conformations. The correlations thus obtained between the SS and CS stretch frequencies and the conformation are useful in understanding similar correlations in proteins containing the disulfide bridge. A simple way is presented to identify local C 2 symmetry in a disulfide bridge through Raman polarization studies.

Vibrational studies of the disulfide group in proteins: Part II. Ab initio force fields and normal mode frequencies of dimethyl, methylethyl, and diethyl disulfides

The geometric parameters and quadratic force constants of dimethyl disulfide, methylethyl disulfide, and diethyl disulfide in all their stable conformations and transition state conformations have been obtained from ab initio Hartree—Fock calculations with a 3-21G* basis set. Thirteen scale factors applied to the ab initio force field allow the reproduction of 62 observed frequencies with an average error of 0.5%. Relationships between the SS and CS stretch frequencies and the conformer internal rotation geometry are obtained. The results reported here provide a good basis for further investigation of the vibrational spectra of proteins containing cystine residues.

A b i n i t i o calculations of the vibration–rotation spectrum of HCS−

The three dimensional near equilibrium potential energy and dipole moment surfaces of the electronic ground state of HCS− have been calculated from correlated MCSCF-CI electronic wave functions. These data have been used in perturbation and variational calculations of the bound and electron detachment anharmonic vibration–rotation levels. The electron affinity EA0 is calculated to be 0.41 eV and the equilibrium geometry to be RCH=1.111 Å, RCS=1.687 Å, α=106°. The fundamental vibrational band origins and integrated absorption band intensities are predicted to be 2648 cm−1/1318 cm−2 atm−1 (CH stretch), 1140 cm−1/145 cm−2 atm−1 (bend), and 911 cm−1/50 cm−2 atm−1 (CS stretch) in HCS−. The components of the dipole moment functions are given analytically. The dipole moment in the vibrational ground state of HCS− has been calculated to be 2.122 D. Radiative transition probabilities among low lying vibrational levels have also been evaluated. It is found that the radiative lifetimes vary in a mode-specific way. The theoretical photoelectron spectrum of HCS− and DCS− is reported.

Vibrational radiative lifetimes in H 2Se and HCS −

Vibrational dipole matrix elements and radiative transition probabilities for H 2Se and HCS − have been evaluated from theoretical three-dimensional electric dipole moment and potential energy functions. The radiative lifetimes of the ( v=1, J=0) levels of HCS − have been calculated to be 3.5 ms (CH stretch), 172 ms (HCS bend) and 785 ms (CS stretch), and in H 2Se to be 122 ms (symmetric stretch), 1045 ms (bend) and 101 ms (asymmetric stretch). The lifetimes of higher vibrational states with no extensive mode-mixing decrease with increasing vibrational quantum number and vary in a mode-specific way.

Microwave and Infrared Spectra of Sulfur Dicyanide: Molecular Structure, Dipole Moment, Quadrupole Coupling Constants, Centrifugal Distortion Constants, and Vibrational Assignment

The microwave spectra of S(CN)2, SC13CN2, SC2N15N, and 34S(CN)2 were studied in the frequency region 8 to 30 Gc/sec. Isotope shifts yield the following structural parameters: CS=1.701 Å, ∠CSC=98°22′, CN=1.156 Å, 2φ=108°24′, where 2φ is the angle between the two C–N internuclear lines. Thus the SCN angle deviates from linearity by 5°. From Stark-effect measurements the dipole moment was determined to be 3.04±0.03 D. Quadrupole hyperfine structure partially resolved in several transitions of S(CN)2 and SC2N15N yields the following coupling constants (Mc/sec): S(CN)2 (χAA=−1.51, χBB=0.30, χCC=1.21) SC2N15N (χAA=−1.51, χBB=0.24, χCC=1.27). Centrifugal distortion constants and the inertial defect for a low-lying vibrationally excited state (v4=1) of S(CN)2 were of great value in making the following assignment of normal frequencies: ω1(A1) 2190 cm−1 (CN stretch),ω5(B1) 2180 cm−1 (CN stretch),ω2(A1) 672 cm−1 (CS stretch),ω6(B1) 690 cm−1 (CS stretch),ω3(A1) 378 cm−1 (SCN def.),ω7(B1) 328 cm−1 (SCN def.),ω4(A1) 135 cm−1 (CSC def.),ω8(B2) 372 cm−1 (SCN def.),ω9(A2) (376) cm−1 (SCN def.).Measurements of the infrared spectrum extended to 50 cm−1; ω9(A2) which is infrared inactive, is estimated from an approximate potential function. Vibration—rotation interaction parameters determined from a normal coordinate analysis give inertial defects of 0.4503, 0.4499, 0.4548, and 0.4564 amu Å2 for S(CN)2, SC13CN2, SC2N15N, and 34S(CN)2, respectively. The corresponding observed values are 0.4869, 0.4864, 0.4927, and 0.4920. For the excited state v4=1, the calculated and observed values of the inertial defect are 1.2726 and 1.3837 amu Å2, respectively.