Rotational analysis of the 30¹ band of the Ã6Σ⁺ ← X6Σ⁺ system of CrCCH.

The spectrum of chromium acetylide, CrCCH, has been investigated in the near-infrared region (10,500-14,500 cm(-1)) and an intense band system is observed. The 30¹ band of this system has been investigated at high resolution (0.025 cm(-1)) and this system is identified as the Ã6Σ⁺ ← X6Σ⁺ system, analogous to the near IR spectra of the CrH, CrF, and CrCl molecules. Among the many unidentified lines, we have assigned 341 lines belonging to 43 of the 54 allowed branches. Using combination differences, the rotational constants of the ground state have been determined. Computational results on the ground state are also reported, along with a comparison to other monoligated monovalent chromium compounds.

Structural evidence of anomeric effects in the anesthetic isoflurane.

The conformational and structural properties of the inhalational anesthetic isoflurane (1-chloro-2,2,2-trifluoroethyl difluoromethyl ether) have been probed in a supersonic jet expansion using Fourier-transform microwave (FT-MW) spectroscopy. Two conformers of the isolated molecule were identified from the rotational spectrum of the parent and several (37)Cl and (13)C isotopologues detected in natural abundance. The two most stable structures of isoflurane are characterized by an anti carbon skeleton (τ(C(1)-C(2)-O-C(3)) = 137.8(11)° or 167.4(19)°), differing in the trans (AT) or gauche (AG) orientation of the difluoromethyl group. The conformational abundances in the jet were estimated from relative intensity measurements as (AT)/(AG) ≈ 3:1. The structural preferences of the molecule have been rationalized with supporting ab initio calculations and natural-bond-orbital (NBO) analysis, which suggest that the molecule is stabilized by hyperconjugative effects. The NBO analysis of donor-acceptor (LP → σ*) interactions showed that these stereoelectronic effects decrease from the AT to AG conformations, so the conformational preferences can be accounted for in terms of the generalized anomeric effect.

Electronic spectroscopy and electronic structure of diatomic CrC.

Optical spectra of jet-cooled diatomic CrC have been recorded in the near infrared region using resonant two-photon ionization spectroscopy combined with mass-selective detection of the resulting ions. Several weak transitions have been observed, along with one relatively strong band near 842 nm. Rotational resolution and analysis of this band confirms that the ground state is of (3)Sigma(-) symmetry. Ab initio calculations have been performed that demonstrate that the ground state is highly multiconfigurational in nature, with a leading configuration of 1sigma(2)2sigma(2)1pi(4)1delta(2) for the ten valence electrons. From the rotational analysis of the 842 nm (3)Sigma(-)<--X (3)Sigma(-) band, the derived spectroscopic constants of the ground and excited states for (52)Cr(12)C are B(0)"=0.659 97(49), lambda(0)"=6.74(24), gamma(0)"=-0.066(20), T(0)=11 870.7660(65), B'=0.608 29(39), lambda'=7.11(24), and gamma'=0.144(17) cm(-1). Here and throughout this article, 1sigma error limits are reported in parentheses. These rotational constants may be inverted to provide the bond lengths in the ground and excited states, r(0)"=1.6188(6) A and r'=1.6861(5) A, respectively. Ab initio calculations show that the upper state is the third state of (3)Sigma(-) symmetry.

The conformational landscape of the volatile anesthetic sevoflurane.

Most of the volatile haloorganic compounds used as anesthetics exhibit a heavy-atom frame large enough to allow for conformational changes. Even in the absence of directed intermolecular interactions, only some or just one of the possible conformations might have an appreciable abundance. In this realm, the structure of the anesthetic haloether sevoflurane (CH(2)F-O-CH(CF(3))(2)) has been resolved using Fourier-transform microwave (FT-MW) spectroscopy in a supersonic-jet expansion. In isolated conditions sevoflurane adopts a single conformation characterized by a gauche fluoromethoxy group and a near-symmetric orientation of the isopropyl group with respect to the ether plane (cis H-C(ipr)-O-C(F)). Substitution and effective structures have been calculated from the rotational spectra of all (13)C and (18)O monosubstituted isotopic species observed in natural abundance. The electric dipole moment components were determined from additional Stark effect measurements. The experimental structures and rotational data are compared with those obtained from supporting ab initio predictions using MP2 calculations and the B3LYP hybrid functional.

Vibronic spectroscopy of unsaturated transition metal complexes: CrC2H, CrCH3, and NiCH3.

Vibronically resolved resonant two-photon ionization and dispersed fluorescence spectra of the organometallic radicals CrC(2)H, CrCH(3), and NiCH(3) are reported in the visible and near-infrared wavelength regions. For CrC(2)H, a complicated vibronic spectrum is found in the 11 100-13 300 cm(-1) region, with a prominent vibrational progression having omega(e) (')=426.52+/-0.84 cm(-1), omega(e) (')x(e) (')=0.74+/-0.13 cm(-1). Dispersed fluorescence reveals a v(")=1 level of the ground state with DeltaG(1/2) (")=470+/-20 cm(-1). These vibrational frequencies undoubtedly pertain to the Cr-C(2)H stretching mode. It is suggested that the spectrum corresponds to the A (6)Sigma(+)<--X (6)Sigma(+) band system, with the CrC(2)H molecule being linear in both the ground and the excited state. The related CrCH(3) molecule displays a vibronic spectrum in the 11 500-14 000 cm(-1) region. The upper state of this system displays six sub-bands that are too closely spaced to be vibrational structure, but too widely separated to be K structure. It is suggested that the observed spectrum is a (6)E<--X (6)A(1) band system, analogous to the well-known B (6)Pi<--X (6)Sigma(+) band systems of CrF and CrCl. The ground state Cr-CH(3) vibration is characterized by omega(e) (")=525+/-17 cm(-1) and omega(e) (")x(e) (")=7.9+/-6 cm(-1). The spectrum of NiCH(3) lies in the 16 100-17 400 cm(-1) range and has omega(e) (')=455.3+/-0.1 cm(-1) and omega(e) (')x(e) (')=6.60+/-0.03 cm(-1). Dispersed fluorescence studies provide ground state vibrational constants of omega(e) (")=565.8+/-1.6 cm(-1) and omega(e) (")x(e) (")=1.7+/-3.0 cm(-1). Again, these values correspond to the Ni-CH(3) stretching motion. (c) 2004 American Institute of Physics.