Rotational Spectra of Acetylenes

In this experiment, several vibrational-rotational infrared bands of C2H2 and C2D2 will be recorded at medium to high resolution ( 1 cm ). These spectra will be analyzed to extract rotational constants for use in the calculation of accurate values for the C—H and C—C bond lengths. The role of symmetry and nuclear spin in determining the activities 

From the harmonic-oscillator model of quantnm mechanics, the term valne G for the vibrational energy levels for a linear polyatomic molecnle can be written as  

Vibrational energy levels and relevant infrared-active transitions for acetylene. 

The fundamental 1 3 and V5 transitions are indicated with bold arrows. 

The set of quantum numbers of a level also serves to define the corresponding wave-function, which in the usual approximation is written as a product of one-dimensional harmonic oscillator functions, 

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E Kostyk, HL Welsh. High resolution rotation-vibration Raman spectra of acetylene. I. The spectrum of C2H2. Can J Phys 58 534-543, 1980. 

Aldritch, P. D., Kukolich, S. G. and Campbell, E. J. (1983) The structure and molecular properties of the acetylene-HCN complex as determined from rotational spectra, J. Chem. Phys. 78, 3521-3530. 

H n.m.r. spectra of complexes of unsymmetrical acetylenes and phosphines show coupling to two nonequivalent phosphorus atoms showing that the square planar environment is maintained in solution 3>. However rotation of both ethylene and acetylene in zerovalent complexes has been predicted 115T... 

Figure 4.16 Left-hand side R(7) of the fundamental acetylenic C—H stretch rovibrational spectra of (CHjljCC CH (above) and (CHjljSiC CH (below). Right-hand side R(5) of the overtone acetylenic C—H stretch rovibrational spectra of (CH3)3CC H (above) and (CH3)3SiC CH (below). In all four cases the measured rotational line and a nonlinear least-squares fit to a single Lorentzian are shown in the upper traces, while residual of the Lorentzian fit and the zero line are shown in the lower traces. (For the sake of clarity upper and lower traces are staggered.) The residuals indicate a true Lorentzian line shape for the carbon compound as expected for the statistical regime of IVR. For the silicon compound the fit to a single Lorentzian is not as exact. The small residuals at the low-frequency side for Si compound (below) in both the fundamental and the overtone are likely due to two isotopes of Si with 4.67% and 3.1 % natural abundance or to a hot-band transition (Kerstel et al., 1991).

H Finsterholzl, G Hochenbleicher, G Strey. Intensity distribution in pure rotational Raman spectra of linear molecules in the ground and vibrational 11 states. Application to acetylene. J Raman... 

N.m.r. investigations include long-range proton-proton coupling in phenylacetylene derivatives spectra of a series of halogenoacetylenes and aromatic acetylenes rotational conformations in the diastereoisomers of 1,3-di-t-butylpropargyl 2-phenylpropionate and spectra as applied to the products (227) and (228) from the cyclobutene (229) and phenyl-ethynylmagnesium bromide... 

Linear molecules have / , the moment of inertia about the molecular axis, equal to zero and equal to f, which are the moments of inertia about two axes perpendicular to the molecular axis and to each other. Polyatomic linear molecules and diatomic molecules have identical rotational energy equations. However, pure infrared rotational spectra can only be observed for those molecules which possess a permanent dipole moment. In carbon dioxide and acetylene, for example, the permanent dipole moment is zero because of symmetry. 

Fig. 1.28. Gas phase contours of absorption bands of acetylene, acetonitrile, tri-methylamine, and benzene measured in a 10-cm cell with a rock salt prism, (a) Acetylene at 81 mm pressure. The parallel and perpendicular bands are marked. The perpendicular band has a central peak which is absent in the parallel bands. There is partial resolution of some rotational fine structure, (b) Acetonitrile at 67 mm Hg pressure. The moment of inertia about the symmetry axis is relatively small. The perpendicular band has a round contour with some fine structure resolved. The parallel band is a triplet, (c) Trimethylamine at 27 mm Hg pressure. The parallel bands are a little wider than the perpendicular bands and show a more distinct central peak. The triplet bands are asymmetrical because of centrifugal distortion, (d) Benzene at 26 mm Hg pressure. The parallel and perpendicular bands are not very different. The references cited refer to the band assignments the spectra shown were obtained by the authors of this text.

Comparison of emission spectra between 2100 A and 6500A has shown only small differences in relative concns of excited species between low-pressure diffusion flames and explns, whereas during explns peak intensities may be as much as 100 times greater. The time dependence of the free-radical emission during expln indicates the formation sequence to be OH, CH, C2, and evidence for the forbidden CO Cameron bands has been obtained. Similarly the ultraviolet absorption spectrum of the OH radical in acetylene— H2—02 detonations has been measured in conjunction with the associated rarefaction waves (Ref 7). Analysis of the absorption spectrum has indicated average rotational temps greater than 3000°K during the initial 310 microseconds... 

The oldest of the spectroscopic radiation sources, a flame, has a low temperature (see Section 4.3.1) but therefore good spatial and temporal stability. It easily takes up wet aerosols produced by pneumatic nebulization. Flame atomic emission spectrometry [265] is still a most sensitive technique for the determination of the alkali elements, as eg. is applied for serum analysis. With the aid of hot flames such as the nitrous oxide-acetylene flame, a number of elements can be determined, however, not down to low concentrations [349]. Moreover, interferences arising from the formation of stable compounds are high. Further spectral interferences can also occur. They are due to the emission of intense rotation-vibration band spectra, including the OH (310-330 nm), NH (around 340 nm), N2 bands (around 390 nm), C2 bands (Swan bands around 450 nm, etc.) [20], Also analyte bands may occur. The S2 bands and the CS bands around 390 nm [350] can even be used for the determination of these elements while performing element-specific detection in gas chromatography. However, SiO and other bands may hamper analyses considerably. 

Recent experimental studies (Reisner et al., 1984 Abramson et al., 1985 Smith et al., 1987) have focused on the high-resolution spectroscopy of vibrationally excited molecules, and these studies have provided valuable insight into the nature of vibrational and vibrational/rotational energy levels as the energy is increased. Two of the most extensively studied molecules are acetylene and formaldehyde, and the properties of their spectra are summarized below. 

A full account bas now appeared of the reaction of dihydrobi (n -Qrclo-pentadienyl)molybdenuin with perfluorobut-2-yne and other acetylenes containing electron-withdrawing substituents cf. Vol. 1, p. 163). Insertion of perflaorobutyne into one Mo—bond gives two isomers [(3a and (3b), see Figure 1 ], both shown by their n-mj. spectra to have trans-CFi groups variable-temperature n.m.r. studies show the barrier to interconversion to be ca. 71 kj mol, but do not distinguish between rotation about the... 

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