Molecular Spectra and Molecular Structure - Lec 31S

Lecture # 31 Supplement Contents 1. 2. 3. 1. “Analysis of the Near Ultraviolet Absorption Spec­ trum of Acetylene” by K. Keith Innes . . . . . . . “The Bent Excited State of Acetylene” by G. W. King and C. K. Ingold . . . . . . . . . . . . . . . . “Nonclassical Double-Bridged Structure in SiliconContaining Molecules: Experimental Evidence in Si2 H2 from Its Submillimeter-Wave Spectrum” by M. Bogey, H. Bolvin, C. Demuynck, and J. L. Destombes . . . . . . . . . . . . . . . . . . . . . . . 1 2 2 “Analysis of the Near Ultraviolet Absorption Spectrum of Acetylene” by K. Keith Innes J. Chem. Phys. 22, 863-876 (1954). ABSTRACT The absorption spectrum of acetylene in the region 1970-2470Å has been photographed in the fourth and fifth orders of a twenty-one-foot grating spec­ trograph with absorbing paths of 0.007 to 70 m-atmos. It has been possible to extend the arguments which led King and Ingold to conclude that in the upper electronic state the molecule is bent and has C2h symmetry. The upper electronic state is Au or, possibly Ag . The effective moment of inertia Ia of the excited state is small, causing sub-band separations of 50 to 150 cm−1 . These sub-bands all have P , Q, and R branches, are well resolved, and have the appearance of bands of diatomic molecules. The following rotational constants of the upper state are derived: A0 = 12.94, B0 = 1.1247, C0 = 1.0297 cm−1 . If the C–H distance is assumed to be between 1.070 and 1.090Å, one finds �CCH = 120◦ . A, rCC = 1.388 ˚ Ground-state combination differences are in agreement with precise values de­ rived from infrared data. Some new lower state B values are obtained. The O–O vibrational transition is shown to be at 42,197.69 cm−1 . The subbands form ν4�� progressions and ν3� progressions. Analysis gives two upper state vibrational frequencies ν3 (CCH) = 1047.70 cm−1 1 ν2 (CC) = 1389 cm−1 . The following lower state vibrational constants are obtained: ω40 = 608.26 , x44 = +3.29, g44 = +0.17 cm−1 . The fundamental frequency ν4�� is the sum of these constants, that is, 611.72 cm−1 . 2. “The Bent Excited State of Acetylene” by G. W. King and C. K. Ingold Nature 169, 1101-1102 (1952). FIRST PARAGRAPH The band systems of C2 H2 and C2 D2 at 2400–2100 A. have been studied in absorption, and the vibrational and resolved rotational structures fully analysed. It emerges that, in its excited state, acetylene is a plane centrosymmetric zig-zag (like ethylene from which two trans-related hydrogen atoms have been removed). It is therefore an asymmetric top. Its dimensions in the lowest vibrational level of that state are CC = 1 · 383 A., CH = 1 · 08 A., ∠CCH = 120 · 2◦ . The frequency of the CC stretching vibration is 1,385 cm−1 in C2 H2 and 1,300 cm−1 in C2 D2 , the derived mean force-constant being F = 7 · 2 × 105 dyne/cm.; while the vibration which together opens the angles has the frequency 1,049 cm−1 in C2 H2 and 835 cm−1 in C2 D2 , the corresponding mean bending-force constant being δ = 8 · 5 × 10−12 dyne.-cm./radian. The electronic wave function has the two-fold axis and plane, but not the centre of symmetry of the zig-zag molecule. The lowest vibrational level of this bent excited state lies at 42,209 cm−1 in C2 H2 and 42,280 cm−1 in C2 D2 , above that of the linear normal state, the difference of 71 cm−1 arising from differences in zero-point vibrational energy. In the transition from the normal to the excited state, the electrons oscillate perpendicularly to the plane of the bent molecule. The resultant electron-spin remains zero. The transition probability, expressed as an oscillator strength, is 8 × 10−5 . All this follows from the analysis of the spectra. 3. “Nonclassical Double-Bridged Structure in SiliconContaining Molecules: Experimental Evidence in Si2 H2 from Its Submillimeter-Wave Spec­ trum” by M. Bogey, H. Bolvin, C. Demuynck, and J. L. Destombes Phys. Rev. Lett. 66, 413-416 (1991) ABSTRACT The first spectroscopic evidence of a double-bridged silicon molecule is re­ ported. The submillimeter-wave rotational spectrum of the disilyne Si2 H2 has been observed in a low-power silane plasma cooled at liquid-nitrogen tempera­ ture, with argon as buffer gas. These measurements led to the determination of the r0 structure, which is in good agreement with ab initio calculations. This nonclassical structure is confirmed by the observation of the 29 Si and 30 Si monosubstituted forms. 2