Kraitchman has shown that a single isotopic substitution on an atom is sufficient to determine directly the coordinates of that atom with respect to the principal axes of the original molecule. Kraitchman's formulas represent exact solutions of the equations for the equilibrium moments of inertia. However, the effects of the zero‐point vibrations are such that the coordinates obtained by substitution from the ground state moments of inertia I0 are systematically less than r0. These coordinates have here been called r (substitution) or rs, and it is found that rs≃(r0+re)/2, and Is= ∑ imirsi2≃(I0+Ie)/2.In the usual method of solution, the coordinate of one atom is determined from the equation for I0, and therefore the difference I0—Is must be made up by this one coordinate. This introduces a large error in the structures normally determined from ground state constants, and results in variations of 0.01 A in structures determined from different sets of isotopic species. If instead, we obtain the structure on...

A new criterion for the determination of molecular structures from ground state rotational constants

The vast majority of gaseous chemical substances exhibit fundamental vibrational absorption bands in the mid-infrared spectral region (≈ 2–25 µm), and the absorption of light by these fundamental bands provides a nearly universal means for their detection. A main feature of optical techniques is the non-intrusive in situ detection capability for trace gases. The focus time period of this chapter is the years 1996–2002 and we will discuss primarily CW mid-infrared laser spectroscopy. We shall not attempt to review the large number of diverse mid-infrared spectroscopic laser applications published to date. The scope of this chapter is rather to discuss recent developments of mid-infrared laser sources, with emphasis on established and new spectroscopic techniques and their applications for sensitive, selective, and quantitative trace gas detection. For example, laboratory based spectroscopic studies and chemical kinetics, which will also benefit from new laser source and technique developments, will not be considered.

Mid-Infrared Laser Applications in Spectroscopy

Frequency down-conversion of solid-state laser sources to the mid-infrared spectral range using non-oxide nonlinear crystals

Context. The discovery of amino acids in meteorites fallen to Earth and the detection of glycine, the simplest of them, in samples returned from a comet to Earth strongly suggest that the chemistry of the interstellar medium is capable of producing such complex organic molecules and that they may be widespread in our Galaxy. Aims. Our goal is to investigate the degree of chemical complexity that can be reached in the interstellar medium, in particular in dense star-forming regions. Methods. We performed an unbiased, spectral line survey toward Sgr B2(N) and (M), two regions where high-mass stars are formed, with the IRAM 30 m telescope in the 3 mm atmospheric transmission window. Partial surveys at 2 and 1.3 mm were performed in parallel. The spectra were analyzed with a simple radiative transfer model that assumes local thermodynamic equilibrium but takes optical depth effects into account. Results. About 3675 and 945 spectral lines with a peak signal-to-noise ratio higher than 4 are detected at 3 mm toward Sgr B2(N) and (M), i.e. about 102 and 26 lines per GHz, respectively. This represents an increase by about a factor of two over previous surveys of Sgr B2. About 70% and 47% of the lines detected toward Sgr B2(N) and (M) are identified and assigned to 56 and 46 distinct molecules as well as to 66 and 54 less abundant isotopologues of these molecules, respectively. In addition, we report the detection of transitions from 59 and 24 catalog entries corresponding to vibrationally or torsionally excited states of some of these molecules, respectively, up to a vibration energy of 1400 cm-1 (2000 K). Excitation temperatures and column densities were derived for each species but should be used with caution. The rotation temperatures of the detected complex molecules typically range from ~50 to 200 K. Among the detected molecules, aminoacetonitrile, n- propyl cyanide, and ethyl formate were reported for the first time in space based on this survey, as were five rare isotopologues of vinyl cyanide, cyanoacetylene, and hydrogen cyanide. We also report the detection of transitions from within twelve new vibrationally or torsionally excited states of known molecules. Absorption features produced by diffuse clouds along the line of sight are detected in transitions with low rotation quantum numbers of many simple molecules and are modeled with ~30–40 velocity components with typical linewidths of ~3–5 km s-1 . Conclusions. Although the large number of unidentified lines may still allow future identification of new molecules, we expect most of these lines to belong to vibrationally or torsionally excited states or to rare isotopologues of known molecules for which spectroscopic predictions are currently missing. Significant progress in extending the inventory of complex organic molecules in Sgr B2(N) and deriving tighter constraints on their location, origin, and abundance is expected in the near future thanks to an ongoing spectral line survey at 3 mm with ALMA in its cycles 0 and 1. The present single-dish survey will serve as a solid basis for the line identification and analysis of such an interferometric survey.

/pdf/complex-organic-molecules-in-the-interstellar-medium-iram-30-1xj4x1o7z6.pdf

Complex organic molecules in the interstellar medium: IRAM 30 m line survey of Sagittarius B2(N) and (M)

This article provides a review of recent work in the field of helium nanodroplet spectroscopy with an emphasis on the dynamical aspects of the interactions between molecules in helium as well as their interaction with this unique quantum solvent. Emphasis is placed on experimental methods and studies introducing recent new approaches, in particular including time-resolved techniques. Corresponding theoretical results on the energetics and dynamics of helium droplets are also discussed.

http://iopscience.iop.org/article/10.1088/0953-4075/39/8/R01/pdf

Spectroscopy and dynamics in helium nanodroplets

Abstract The following rotational spectra of carbonyl sulfide have been accurately remeasured with a beam-absorption spectrometer: 16O12C32S in the vibrational ground state and in the vibrational state 0 11 0. 16O12C34S in the vibrational ground state. The ground state spectra of the following isotopic species have been measured with a video spectrometer: 17O12C32S, 18O12C32S, 16O13C32S, 16O12C33S, 16O12C36S, and 16O13C34S. Our measurements have enabled us to improve the molecular constants notably for these cases.

The Millimeter Wave Rotational Spectra of Carbonyl Sulfide

The rotational constants of methyl lodide

Spectre de rotation millimétrique du malononitrile

Microwave spectra of methyl chloride, methyl bromide, and methyl iodide in the ν6 = 1 excited vibrational state

The ground-state microwave spectrum of methyl iodide has been measured between 59 and 240 GHz with a molecular beam spectrometer and by use of the Lamb dip method. The following molecular parameters have been accurately determined: B = 7501275.70(2) kHz; D J = 6.3070(2) kHz; D JK = 98.762(3) kHz; H J = −0.0051(6) Hz; H JK = 0.04(2) Hz; H KJ 4.51(3) Hz; eq. Q = −1934.136 (5) MHz; C ⊥ = −17.40(6) kHz; C | = −19.4(5) kHz; x J = −1.41(4) kHz; x K = −38(1) kHz and x d = 26.2(6) kHz. The last three constants characterize the interaction of centrifugal distortion with quadrupole coupling. The data are used to determine the elements of the iodine nuclear magnetic shielding tensor. Comparisons of the shielding are made for molecules which contain one iodine atom.

Doppler-free rotational spectrum of methyl iodide. Nuclear quadrupole, spin-rotation and nuclear shielding tensors of iodine

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