to the first overtone. For higher overtones, there is an approximate order of magnitude decrease for each quantum level of excitation.Line Widths. The simplest picture of overtone spectra would produce only one transition for each C-H upper state (ul = 0 -v1 = 1, 2, 3, 4, 5, 6, ...). In reality one has additional absorption bands close to the main absorption. If there is a high density of vibrational and combination states and a substantial fraction of them interact resonantly, the vibrational spectrum will consist of broad overlapped bands in which the secondary transitions borrow some intensity from the main transition. The nature of the coupling between states will determine the overall appearance of the vibrational spectrum. For this particular molecule, CF3C= CH, the overtone absorptions involving the C-H stretch show only (at each side and separated from the main absorption) bands due to sum and difference transitions which originate from interactions with low-frequency modes, for example, the transitions found for (vu1 f vl0) and (vuI f v7). Also, the hot-band transition (u7 -(v7, vv,)) which accompanies the main absorption (0 -uul) increases its separation in energy from the main band as the quantum number of the upper level increases. The C-H vibrational overtone spectra obtained for CF3C=CH in the present investigation do not show resonant interactions in which the vibrational bands become broad due to superposition of many vibrational bands with slightly different band centers. An example of this would be saturated molecules like CF3H and (CF3),CH, where one lower molecular energy state is coupled with substantial line strength to several close-lying upper states. The bandwidths observed for C-H absorptions of C F 3 C~C H are relatively constant. Except for the full width a t half-maximum (fwhm) = 55 cm-' observed for u1 = 5 , the fwhm for transitions to u1 = 2, 3, 4, and 6 are between 30 and 35 cm-I, which indicates there is no strong resonant couplings between these states and other vibrational fundamentals or overtones. Also, no apparent intensity enhancement of the combination bands due to resonance between the main absorption and the combination bands is observed. In general, the interactions that give rise to sum and difference bands and the interactions that produce hot bands (v7 -(v7, u v I ) ) are very strong because these bands are observed for all levels studied. Although these couplings are strong, they are not resonant interactions in nature as is the case for saturated compounds. In compounds such as CF3H and (CF,)$H there is a Fermi resonance involving the C-H bending overtone and the C-H stretch. The main reason for the absence of this Fermi resonance in C F 3 G C H is the energy mismatch between the overtone of the bending mode and the C-H stretch.
ConclusionsThe spectrum of C-H and CEC overtones of 3,3,3-trifluoropropyne has been investigated with intracavity photoacoustic spectroscopy and standard infrared techniques. Three types of vibrational transitions are identified for C-H abso...