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The vibrational spectra of Fe(12C16O)5, Fe(13C16O)5, and Fe(12C18O)5 have been observed. Most of the fundamentals have been assigned, with some changes from earlier results in the literature. Potential constants have been calculated by constraining many of the interaction constants to values estimated from analogous constants of the hexacarbonyls. The results indicate that the axial CO bonds are stronger than the equatorial CO bonds. This implies that the axial MC bonds are weaker than the equatorial MC bonds, though the calculated MC potential constants are not significantly different.

A total of 44 bands in the infrared and Raman spectra of 238UF6 vapor has been measured using long-path absorption cells and multiple-reflection Raman cells. The types of contours exhibited by these bands, and analogous contours in the spectrum of SF 6 , are discussed. Overtone and combination bands with resolved Q branches are used to estimate the anharmonicity corrections for the stretching fundamentals. The infrared contours of 1'3 (at 228 K) and 1'4 (at 256 K) have yielded estimates for the Coriolis constants S3 and S4 that satisfy the sum rule. The 235 UF 6 -238 UF 6 frequency shifts in these fundamentals have also been measured. All of these data, plus vibrational amplitudes from the electron diffraction study of Seip, have been combined to determine the general quadratic harmonic force field of UF 6 . Only three valence force constants are appreciably different from zero: f, = 3.85 ± 0.05, f~ ~ fa -fa'a =0.12 ± 0.0 1, and f" (cis interaction) = 0.30 ± 0.02, all in mdyn/ A. The large differences between this force field and that previously determined for WF6 are attributed to the effects of f -orbital contributions in UF 6 •

An accurate closed-form expression for the rotational partition function of linear polyatomic molecules in Il: electronic states is derived, including the effect of nuclear spin (significant at very low temperatures) and of quartic and sextic centrifugal distortion terms (significant at moderate and high temperatures). The proper first-order quantum correction to the classical rigid-rotator partition function is shown to yield Qr -;:;;/3 -I exp(/3 13), where/3=hcB IkTandB is the rotational constant in cm-I ; for /3>0.2 additional power-series terms in/3 are necessary. Comparison between the results ofthis treatment and exact summations are made for HCN and C z H 2 at temperatures from 2 to 5000 K, including separate evaluation of the contributions of nuclear spin and centrifugal distortion.

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Doppler-limited tunable diode laser spectra of v. of 28SiF. have been analyzed and the spectroscopic constants determined. In contrast to most earlier low-resolution studies, the Coriolis constant t •. when combined with t 3 as obtained from previous laser spectroscopy. yields a zeta sum that is within 5% of the expected harmonic value of 1/2. The band origins of 12 overtones and combinations have been obtained from Fourier-transform spectra (0.04 cm-1 resolution). resulting in estimates of the anharmonicity constants and harmonic frequencies. From the Coriolis constants and the isotope shifts in v 3 we have redetermined the general quadratic force field of SiF 4 ,

A simple, accurate closed-form expression for the rotational partition functions of symmetric-top molecules is derived, which includes the effects of nuclear-spin statistics (significant at very low temperatures), quartic and sextic centrifugal distortion terms (moderate and high temperatures), and inversion (all temperatures): Qr≂σ*(πm)1/2  exp[β(4−m)/ 12]β−3/21+β2(1−m)2/90+⋅⋅⋅](1+δ) (1+ρ1β−1+ρ2β−2+ρ3β−3), where β≡hcB/kT, m≡B/A for prolate tops or B/C for oblate tops (A>B>C are the rotational constants), and σ*≡Πi(2Ii+1)/σ, where Ii is the spin of nucleus i and σ is the classical symmetry number. For C3vXY3 and WXY3 molecules, the low-temperature nuclear-spin correction is δ≊2(2IY+1)−2 exp[π2m(βm−6)/54β]; similar expressions are given for other molecular models. The ρ’s are functions of the centrifugal distortion constants DJ, DJK, DK, HJ, etc. If inversion doubling occurs, with a splitting ΔG in the ground vibrational state, this expression for Qr is to be multiplied by (1−βΔG/2B+⋅⋅⋅). The molecules NH3, CH3D, CHD3, allene (C3H4), and ethane (C2H6), including isotopic variants, are discussed and illustrative calculations presented for T=10 to 2000 K.