S.N. Thakur

The benzene ground state potential surface. I. Fundamental frequencies for the planar vibrations

The accuracy of vapor phase vibrational data has been improved for all 12 deuterium‐labeled benzenes and for 13C12C5H6 and 13C6H6. Many vapor phase fundamental frequencies are observed for the first time. Precise isotopic frequency/splitting patterns for ν1, ν18, and ν19 have been obtained. Isotope induced harmonic mode mixing matrices are given for all 14 labeled benzenes and used to provide detailed description of the fundamental bands observed in the spectra. These descriptions provide numerous reassignments for the fundamental bands, particularily in low symmetry deuterium benzenes. The matrices show that some skeletal modes, such as ν1, gain CH stretching character as a result of deuterium labeling, providing a rationalization for the increased anharmonicity observed in recent jet experiments for C6D6. In addition, a reassessment of Fermi resonance gives 3072.3 cm−1 for the unperturbed frequency (correction +24 cm−1) for the e1u mode ν20 in C6H6 refining the CH local mode anharmonic constant, 2xii, t...

Vibrational assignments in the 288 nm Ã1A′-X̃1A′ electronic absorption spectrum of styrene: New information on the C(1)-C(α) torsional vibrational levels in the ground electronic state of styrene, styrene-β-D2, and p-fluorostyrene

Abstract The 288 nm A - X absorption spectra of styrene and styrene-β-D 2 have been photographed at high resolution and vibrationally analyzed. The vibronic parent band structure resembles quite strongly that of A - X systems of aniline, phenol, chlorobenzene, and ethynylbenzene and suggests that the electronic transition involves primarily the benzene ring. However, the fact that the polarization direction of the electronic transition moment is rotated by about 90° relative to that in the above four molecules indicates some participation of the substituent. No vibrational evidence has been found to support the proposal by Hui and Rice ( 5 ) that the CH 2 -group of the substituent is perpendicular to the plane of the rest of the molecule in the A state. It can be argued, though, that the A - X system may not show such evidence even if this were the configuration. On the other hand the observed small shift of +19 cm −1 of the 0 0 0 band from styrene to styrene-β-D 2 is incopatible with such a structure. Quite novel in a spectrum of this type is the activity of ν 42 , the C (1)- C ( α ) torsional vibration. It is found to be very heavily mixed in the A state with ν 41 , an out-of-plane vibration which is principally a motion of the substituent as a whole. As a result of this, and the very low wave-number of ν 42 in the X state, the cross-sequence 41 0 1 42 1 0 , 41 0 2 42 2 0 , … is very prominent and stronger than the sequence 42 1 1 , 42 2 2 …. Interpretation of the intense cross-sequence, together with 41 1 1 and 42 1 1 assignments, results in values for the separations of the first three vibrational levels of ν 42 in the X state. Quantitatively these rely on the far infrared gas-phase value for ν 41 in the X state but the v 42 = 1-0, 2-1, and 3-2 separations are close to 43, 53, and 58 cm −1 in styrene, 37, 46, and 51 cm −1 in styrene-β-D 2 and 44, 52· 5 , and 57 cm −1 in p -fluorostyrene. The values for styrene are at variance with those obtained by Carreira and Towns ( 26 ) from their interpretation of the Raman spectrum and suggest that the problem of the torsional potential function for ν 42 in the X state of styrene may not yet be solved.

Overtone spectroscopy of benzene derivatives using thermal lensing

Abstract The vibrational overtones of CH stretching oscillators are reported as observed by conventional IR spectroscopy and dual-beam thermal lensing spectroscopy for benzene, fluorobenzene, chlorobenzene, bromobenzene and benzonitrile in the liquid phase at room temperature. The stretching frequency ωe, the anharmonicity constant ωeχe and the change in CH bond length on substitution in benzene have been determined for all these molecules under the local-mode approximation. Effects of substitution on the change in CH stretching frequency have been discussed in terms of the electronegativity of the substituents as well as the inductive part of the Hammett σ. Variation of thermal lensing signal with chopping frequency and laser power has also been studied.

Rotational band contour analysis in the 3120 Å system of naphthalene

The rotational contour of the 0 + 435 cm-1 (440 1) type B band of the A1B2u - X1Ag system of naphthalene has been analysed by the method of computer simulation. The resulting rotational constants are considerably different from previously reported values. With the new values agreement between the observed 00 0 band contour and a computed type A rather than a type C contour is now clear-cut, confirming previous assignments. The wavenumber of the rotational origin of the 00 0 band is shown to be 32018·53 ± 0·04 cm-1 and that of the 440 1 band is 32453·50 ± 0·04 cm-1.

Quantitative analysis of gallstones using laser-induced breakdown spectroscopy

The utility of laser-induced breakdown spectroscopy (LIBS) for categorizing different types of gallbladder stone has been demonstrated by analyzing their major and minor constituents. LIBS spectra of three types of gallstone have been recorded in the 200-900 nm spectral region. Calcium is found to be the major element in all types of gallbladder stone. The spectrophotometric method has been used to classify the stones. A calibration-free LIBS method has been used for the quantitative analysis of metal elements, and the results have been compared with those obtained from inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements. The single-shot LIBS spectra from different points on the cross section (in steps of 0.5 mm from one end to the other) of gallstones have also been recorded to study the variation of constituents from the center to the surface. The presence of different metal elements and their possible role in gallstone formation is discussed.

Detection sensitivity of laser-induced breakdown spectroscopy for Cr II in liquid samples

The performance of laser-induced breakdown spectroscopy (LIBS) has been evaluated for detection of toxic metals such as Cr in water. Pure aqueous solutions (unitary matrix) with variable Cr concentration were used to construct calibration curves and to estimate the LIBS limit of detection (LOD). The calibration curves for Cr in a binary matrix (Cr plus Cd) and a tertiary matrix (Cr plus Cd and Co) were used to evaluate the matrix effect on the LOD. The LOD for Cr was found to be 1.1, 1.5, and 2.0 ppm (parts in 10(6)) in a unitary, binary, and tertiary matrix, respectively. Once calibrated, the system was utilized for the detection and quantification of the Cr in tannery wastewater collected from different locations in the industrial area of Kanpur, India, where Cr concentrations were determined to be far higher than the U.S. Environmental Protection Agency safe drinking water limit of 0.05 ppm.

Methyl rotor effects on acetone Rydberg spectra. I. The 1A2(3p←n)←1A1 transition

Two‐photon spectra of the acetone 1A2(3px←n)←1A1 Rydberg transition obtained by injection seeded excitation determines a2(ν12) and b1(ν17) acetone 1A2 Rydberg state torsional fundamental frequencies as 70.8 cm−1 (ν’12) and 129.4 cm−1 (ν’17) (compared to the ground‐state values 77.8 and 124.5 cm−1, respectively). Corresponding values in (CD3)2CO are 49.1 and 98.2 cm−1, compared to 53.4 and 96.0 cm−1 in the ground state. The 1A2 state 2ν’12, 3ν’12, and ν’12 + ν’17 energies are also assigned allowing determination of excited‐state potential constants. There are large changes (65–200 cm−1) in these constants from those for the ground state causing significant perturbation in the shape of the 1A2 state potential from that of the ground state. The 1A2 constants are V3=551, V33=349, V’33=−221, and V6=0 cm−1. The effective potential barrier height to internal rotation (202 cm−1) is decreased from the ground state by about 15%, i.e., by ≊35 cm−1; but the barrier height for eclipsed–eclipsed→staggered–staggered syn...

Two-photon photoacoustic spectroscopy of acetone 3p Rydberg states

Abstract Two-photon photoacoustic spectra of acetone- h 6 and − d 6 have been recorded using a pulsed laser in the 3p←n Rydberg transition region (332.2–338.7 nm for two-photon excitation). Comparison of the acetone- h 6 and − d 6 photoacoustic and two-photon resonance enhanced multiphoton ionization spectra shows an intensity reversal between the (3p y ←n) 1 A 1 and (3p x ←n) 1 A 2 transition origins. The dramatic increase in the 1 A 1 state - and to a lesser extent for the 1 B 2 state - photoacoustic signals requires efficient radiationless/dissociative channels for these states, not present for 1 A 2 . The intensity effects are interpreted through the 1 A 1 state having substantial π*←π valence character in contrast to almost pure Rydberg character for the 1 A 2 state.

The electronic emission spectrum of benzaldehyde vapour

Abstract The electronic emission spectrum of benzaldehyde vapour excited in a discharge consists mainly of a 3 A″-X 1 A′ phosphorescence but also contains some weak A 1 A″-X 1 A′ fluorescence. The spectrum has been photographed under high resolution with a view to establishing which a″ vibrations are active in the a-X system. We have shown, with the help of rotational band contours, that the vibration in which the substituent as a whole moves out-of-plane is weakly active and that the substituent torsional vibration is more weakly active. These are the only a″ vibrations which are active with sufficient intensity to be observed in the spectrum. In the A-X fluorescence an a″ wagging vibration of the hhydrogen atoms of the phenyl ring is moderately strongly active, as is the corresponding excited state vibration in the A-X absorption system

Photoacoustic Studies on Neutron-Irradiated Rare Earth Oxide Powers

The effect of neutron irradiation on the photoacoustic spectra of Er2O3 and Nd2O3 in powder form has been studied. The irradiation shows a quenching effect on the photoacoustic signal which regains its intensity after a lapse of some time. It is believed that the irradiation changes the oxidation state of the rare-earth ions from M3+ to M2+. The half-life of the recovery process Er2+ to Er3+ in the case of irradiated Er2O3 is found to be 46 h.