G. N. R. Tripathi

The resonance Raman spectrum of phenoxyl radical

The Raman spectra of phenoxyl, phenoxyl‐d5, and phenoxyl‐2,4,6‐d3 radicals produced pulse radiolytically in aqueous solutions and observed by time resolved resonance Raman methods are reported. Excitation was mainly in the narrow and moderately intense (0,0) phenoxyl absorption band at 400 nm. These Raman spectra are superimposed on a broad fluorescence in the region of 410–440 nm which is also ascribed to the phenoxyl radical. A very intense Raman band, which is assigned to a mode principally involving the CO stretch (Wilson 7a), is observed at 1505 cm−1. In the fully and partially deuterated radicals this band is at 1489 and 1487 cm−1, respectively, indicating that in the latter instance the vibration is shifted toward a lower frequency, probably by Fermi resonance with an underlying weak vibration. In the protonated radical moderately intense bands are also observed at 990 and 528 cm−1. The higher of these, by virtue of its considerably lower frequency in the deuterated radicals, is assigned to a CH bending mode (Wilson 18a). The other is affected very little by the substitution and is assigned to a CCC bending mode (Wilson 6a). Weak bands at 1157, 1056, and 865 cm−1 in phenoxyl, phenoxyl‐2,4,6‐d3, and phenoxyl‐d5 radicals are assigned to a second CH bending mode (Wilson 9a). Two weak bands are also reported at 1398 and 1331 cm−1 and are attributed to the nontotally symmetric 19b and 14 modes. No Raman band is, however, observed in the 1550–1650 cm−1 region where the Wilson 8a ring stretching motion prominent in semiquinone radicals is expected. The excitation profile of the 1505 cm−1 emission largely follows the narrow absorption spectrum of the radical in the region of 400 nm. Below 390 nm, where the electronic excitation in resonance is to an upper vibrational level, resonance enhancement of the Raman signals is reduced by more than an order of magnitude. The decay of the radical at 10−4 M, as monitored by its Raman signal, corresponds to its loss mainly in second order processes having a rate constant of (2.6±0.3)×109 M−1 s−1, in agreement with measurements by absorption methods at a 50‐fold lower concentration.The Raman spectra of phenoxyl, phenoxyl‐d5, and phenoxyl‐2,4,6‐d3 radicals produced pulse radiolytically in aqueous solutions and observed by time resolved resonance Raman methods are reported. Excitation was mainly in the narrow and moderately intense (0,0) phenoxyl absorption band at 400 nm. These Raman spectra are superimposed on a broad fluorescence in the region of 410–440 nm which is also ascribed to the phenoxyl radical. A very intense Raman band, which is assigned to a mode principally involving the CO stretch (Wilson 7a), is observed at 1505 cm−1. In the fully and partially deuterated radicals this band is at 1489 and 1487 cm−1, respectively, indicating that in the latter instance the vibration is shifted toward a lower frequency, probably by Fermi resonance with an underlying weak vibration. In the protonated radical moderately intense bands are also observed at 990 and 528 cm−1. The higher of these, by virtue of its considerably lower frequency in the deuterated radicals, is assigned to a CH be...

Time-resolved resonance raman spectra of dihalide radical anions in aqueous solution☆

Abstract Time-resolved resonance Raman spectra of pulse radiolytically produced dihalide radical anions in aqueous solution are reported. The harmonic frequency constants (ωc) for , and are found to be 273, 167 and 115 cm−1. The observated anharmonicities are low, implying bond dissociation energies of 1.6, 1.3 and 0.9 (±0.2) eV, respectively.

Resonance Raman scattering of semiquinone radical anions

The time resolved electronic absorption spectra of p‐benzosemiquinone (PBSQ) and 2,5‐di‐t‐butyl‐p‐benzosemiquinone (DTBSQ) radical anions are obtained in pulse radiolysis. The same radicals have been generated chemically in a flow system. The Raman spectra are excited by tuning a pulsed dye laser in the electronic absorption region. Time adjusted gate electronics have been used for the temporal resolution of the Raman spectra from fluorescence. The resonance effect has been utilized to record the Raman spectra of unstable semiquinone radical anions, 10−3–10−4 M in concentration. The observed Raman spectra and excitation profile are interpreted to assign the electronic and the vibrational transitions in the radicals. The semiquinone radical anions are fairly stable on photoirradiation above 330 nm.

Time resolved resonance Raman spectra of anilino radical and aniline radical cation

We report, in this paper, submicrosecond time resolved resonance Raman spectra of anilino radical and its radical cation as observed in pulse radiolytic studies of the oxidation of aniline in aqueous solution. By excitation in resonance with the broad and weak electronic transition of anilino radical at 400 nm (e∼1250 M−1 cm−1) we have observed, for the first time, the vibrational features of this radical. The Wilson ν8a ring stretching mode at 1560 cm−1 is most strongly resonance enhanced. The ν7a CN stretching band at 1505 cm−1, which is shifted to higher frequency by 231 cm−1 with respect to aniline, is also prominent. The frequency of this latter mode indicates that the CN bond in the radical has considerable double bond character. The Raman spectrum of aniline radical cation, excited in resonance with the ∼425 nm electronic absorption (e∼4000 M−1 cm−1), shows features which are similar to phenoxyl radical. Most of the observed frequencies of this radical in solution are in good agreement with vibrati...

The vibrational factor group splittings in crystalline aniline

Evidence for intermolecular dynamic coupling in the molecular vibrations of crystalline aniline has been obtained for the first time. The Raman intensity patterns in polycrytalline aniline at 77 K and the infrared polarization in the oriented aniline films at 40 K are investigated. The crystal spectra are interpreted to deduce the probable site (C1) and factor group (D2h). Some of the earlier vibrational assignments are revised.

Polarized infrared spectra of oriented polycrystalline films. Intermolecular vibrational coupling in crystalline 2,4,6‐tribromoaniline

Highly oriented crystal films of 2, 4, 6‐tribromoaniline have been grown on cesium iodide substrates and their polarized infrared spectra recorded. In addition, the infrared and Raman spectra of this compound have been measured on bulk solid and solution samples. The factor group splittings arising from the intermolecular vibrational couplings in the crystal have been resolved and assigned and the crystal spectra correlated with the ‘‘isolated molecule’’ spectra according to their various symmetries. The interplane angle of the NH2 and phenyl groups is estimated as about 46° in the crystal. An earlier vibrational assignment has been modified to be consistent with the new experimental data obtained.

Resonance Raman spectra of some radiolytically prepared halogen derivatives of para-benzosemiquinone radical anion

The resonance Raman spectra have been obtained on radiolytically and chemically prepared halogen derivatives (chloro‐, bromo‐, 2.5 dichloro‐, tetra chloro‐, and tetra bromo‐) of p‐benzosemiquinone radical anion. Excitation is in the moderately intense absorption band at 430–460 nm. All Raman spectra show a strongly resonance enhanced and polarized line corresponding to a vibrational frequency of 1590–1620 cm−1 which is assigned to the Wilson phenyl mode 8a (CC stretch). A number of weaker lines are also observed and their assignment discussed. The electronic transitions in resonance are identified as 2B3g–2B1u (in D2h point group) in view of the resonance Raman band intensities. This supports the assignment by Harada based on ASMO CI calculations which has recently been in dispute.

Time-resolved Raman observation of the aqueous O3−☆

Abstract A Raman study of the O − +O 2 →O 3 − reaction in aqueous solution is reported. The reaction half-period was measured as ∼ 200 ns in oxygen saturated solution at room temperature. Spectroscopic evidence for the O 3 − -solvent coupled vibrations has been found. The harmonic frequency ( Ω e ) of the O 3 − symmetric stretch in aqueous solution, 1058 cm −1 , is 30–40 cm −1 higher, and the anharmonicity constant ( Ω e χ e ), 9 ± 2 cm −1 , about twice as large as compared to the O 3 −1 inbedded in crystalline matrices or bonded to a counter ion. The O 3 − →2O+O − dissociation energy is drastically lowered on hydration.

Crystal spectra and vibrational assignments in α‐resorcinol

The infrared and Raman spectra of α‐resorcinol crystals are investigated at low temperatures. Fine structure in the vibrational exciton states are resolved. Symmetry assignments of the crystal and molecular transitions are proposed based on the polarization properties of the bands. Most of the earlier assignments are revised.

Para phenylenediamine radical cation structure studied by resonance Raman and molecular orbital methodsa)

The vibrational and electronic structures of p‐phenylenediamine radical cation have been studied using time‐resolved resonance Raman spectroscopy and molecular orbital calculations. The Raman spectra in aqueous solution show striking variations in intensity profile when excitation is varied from 340 to 480 nm. Excitation in resonance with the 460–480 nm absorption shows enhancement of only totally symmetric vibrations. Most prominent are the δNH2 scissor mode at 1658 cm−1 and the Wilson modes ν8a (CC stretch) at 1644 cm−1, ν7a (CN stretch) at 1423 cm−1, and ν9a (CH bend) at 1184 cm−1. Also observed are ν1 (ring breathe) at 840 cm−1 and ν6a (CCC bend) at 467 cm−1. With excitation in the 340–410 nm region the ν1 band becomes relatively stronger and an additional band at 1524 cm−1 appears. This latter band, which dominates the spectrum at 360 nm, is assigned to the nontotally symmetric vibration ν8b (CC stretch) having b3g symmetry that gains intensity through vibronic coupling. Raman spectra of ring‐ and am...