Fourier Transform Absorption Spectra Research Articles

Étude par transformée de Fourier, du spectre, du silane dans la région de 1000 cm−1. Analyse de la diade ν2 et ν4

A Fourier-transform absorption spectrum of the three silicium isotopes of silane in natural abundance have been recorded from 840 to 1040 cm−1. The line width is 3 × 10−3 cm−1 and the absolute accuracy is estimated to be about 2 × 10−4 cm−1 for non-saturated and non-overlapped lines. The analysis of this spectrum is achieved, using a Hamiltonian, formulated with an irreducible tensorial set in the Td group.For the dyad ν2 and ν4 and for each isotopic species, 30 reduced effective Hamiltonian parameters are determined, giving the line wave numbers with 2.5 × 10−4 cm−1 for 28SiH4, 3.4 × 10−4 cm−1 for 29SiH4, and 5.5 × 10−4 cm−1 for 30SiH4 as the standard deviation for J = 0–20.

Fourier-transform electronic spectroscopy in the near-infrared: Torsional barrier of the Ā 1A″ state of nitrosobenzene

The Fourier-transform absorption spectrum of nitrosobenzene vapour in the near-infrared has been recorded, and the vibrational bands near the 869 nm electronic origin of the Ā 1A″ ← X̄ 1A′ (n,π *) transition have been re-analysed. The prominent torsional structure observed is shown to represent sequence transitions in the torsional mode. Electronic excitation leads to an increase of the V 2 barrier hindering internal rotation from roughly 1300 cm −1 in the ground to 2500 cm −1 in the (n,π *) excited state.

Perturbations study of the high-resolution spectrum of methyl bromide in the range of the fundamental band

The Fourier transform absorption spectrum of CH 3Br has been analyzed between 2930 and 3010 cm −1 with a recorded resolution of 0.007 cm −1. More than 1300 lines have been assigned for each isotopic species. Two bands have been identified: the fundamental parallel band ν 1 and the parallel component of ν 3 + ν 5 + ν 6. An anharmonic resonance couples the corresponding levels and affects all the K subbands of these bands. Taking account of this resonance and of a vibrational l-type resonance in ν 3 + ν 5 + ν 6, molecular parameters of these two bands have been determined for the first time. The band centers are (in cm −1) v 1: 2973.184 ( CH 3 79 Br), 2973.183 ( CH 3 81 Br) ; v 3 + v 5 + v 8: 2996.0.094 ( CH 3 79 Br), 2994.864 ( CH 3 81 Br) .

Comparisons of the Carbon-13 Nuclear Magnetic Resonance Spectra of Some Solid Cis- and Trans-Poly(Isoprene)S

Abstract Fourier transforms have been obtained of the 1H and 2H simultaneously noise-decoupled, natural-abundance, 22.6-MHz 13C free-induction decays of solid cis- and trans-poly (isoprene), cis-poly(isoprene-з-d1), and carbon-black-filled cis-poly (isoprene). Carbon-13 line widths and Overhauser enhancements have been measured from the phase-corrected normal and selectively saturated absorption spectra, and spin-lattice relaxation times measured from partially relaxed Fourier transform absorption spectra. The spin-lattice relaxation times and Overhauser enhancements of the individual carbons of the amorphous part of trans-poly (isoprene) are all greater than those of the corresponding carbons of cis-poly (isoprene), and, excluding the deuterated carbon, of cis-poly (isoprene-з-d1). These spin-lattice relaxation times are interpreted in terms of a dipolar relaxation mechanism modulated by the segmental motion of the chain. The greater relative segmental flexibility of the trans-poly (isoprene) chain leads to longer spin-lattice relaxation times. The flexibility of the trans chain also permits more of its segmental motions to satisfy the extreme narrowing conditions, and this leads to the larger observed Overhauser enhancement. The various spin-lattice relaxation times of the individual carbons of cis-poly (isoprene) are not noticeably affected by the addition of carbon black. However, the 13C line widths are affected by the filler. The line widths of individual carbons of the carbon-black-filled cis-poly (isoprene) are greater than those of ordinary cis-poly (isoprene) by factors of 5–10. The broadening is different for different carbons. These results are interpreted in terms of the exclusion by the filler of the polymer from some spatial orientations leading to only partial motional narrowing of the NMR lines. Even in the absence of a filler, factors such as chain entanglements can prevent total isotropic motional narrowing of the lines. This effect can be used to explain the absence of any pronounced line narrowing upon selective deuteration of cis-poly (isoprene), even though large differences in spin-lattice relaxation times result. The 13C line widths of all of these solid, rubbery polymers are as much as a factor of 20 less than the 1H NMR line widths of the same polymers. The narrow lines are attributed to the isolation of the 13C spins from dipolar interactions with each other and with most protons.