Acetonitrile vibrational assignments

The picosecond IR absorption spectrum of the tS state in the fingerprint region is different in w-heptane and in acetonitrile. The spectrum recorded for Si tS in the nonpolar solvent w-heptane is consistent with a species that has a center of symmetry. In acetonitrile, the spectrum exhibits additional weak bands near 1570, 1250, and 1180 cm which are approximately at the same frequencies as strong Raman bands assigned to in-plane vinylic vibrational modes in 5i. This result was taken to suggest a molecular structure for 5i that lacks a center of symmetry in acetonitrile. However, because the intensities of these three bands are weak, it was concluded that either the polarization of 5i or the contribution from polarized S structures to all of the S structures in acetonitrile may be small. 

The emission displayed in Fig. 12b has been assigned as a phosphorescence (triplet singlet emission) due to the relatively long decay time of 12 ps at 80 K in butyronitrile and of 5 ps at 293 K in acetonitrile. The quantum yield is reported to be as high as 30% at room temperature [53]. The structure in the emission spectrum is assigned to vibrational satellites as will be shown below (Sect. 4.2.4). 

The addition of alkali and alkaline earth metal perchlorates to AN causes in the region of the C-N stretching band of AN the appearance of a band produced by acetonitrile molecules in the solvation shells of the cations (fig. 1 for NaClOt solutions) a band of the originally triply degenerate antisymmetric Cl-O stretching vibration splits at increased salt concentration due to ion-aggregate formation (fig. 2, for a solution of LiClO ). The bands (a) and (b) in fig. 2 tire assigned to the free anions, band (1) is due to the vibration of a contact ion pair and band (2) to that of a solvent separated ion pair. Alkaline earth metal perchlorate solutions reveal in addition to the free anion bands (a) and (b) three bands due to [M + C10 )2Y,[M - C10 ]+ and [M + AN)C10 ]+,M + = Mg +,Ca +,Ba +. 

Allyl bromide reacts with Rh(TTP) in acetonitrile to form bright yellow [(o-C3H5)Rh(TTP)Br]PF6. Its molar conductance (160 cmV ohm mole) confirms the electrolyte type (1 1), and the IR spectrum has a weak but well defined band at 1618 cm that is assignable (41) to the C=C stretching vibration of a (r-bonded allyl group. In addition, the band typical of tt allylic structures in the 500-520 cm" region is not observed. Limited solubility prevented NMR studies. 

Comparison of the finger-print region of the IR spectra of irradiated and nonirradiated PPy samples (Fig. 3) reveals the appearance of new sharp bands at 1080 and 1130 cm L The same bands were observed with the films prepared with intentionally added water to acetonitrile during electropreparation of the polymer [4]. These bands were assigned to aC = C- 0- C stretching vibration. It is clear that oxygen is induced to chemically bind to the polymeric structure, after the irradiated electrodes are exposed to air. 

The application of infrared spectroscopy to the solution of such solvation problems is hampered to a certain extent by the fact that complex formation leads to the appearance of new, skeletal vibrations, the coupling of which with the vibrations of the original molecules makes the vibrational spectrum more complicated. The situation may similarly be complicated by other effects, such as changes in the orbital hybridization, back-coordination, etc. In the coordination of the solvent acetonitrile to metal ions, for example, if only the effect of coordination causing a decrease in the electron density on the nitrogen is taken into consideration, the frequency of the C—vibration would be expected to decrease. In fact, in the course of this process the coordination increases the a character of the C—bond, which is accompanied by an increase in the frequency of the C—vibration [Be 61]. In many cases, it is difficult to assign the infrared bands to the corresponding vibrations the conclusions drawn from the spectral data may therefore be uncertain. 

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