Intriguing Electrostatic Potential of CO: Negative Bond-ends and Positive Bond-cylindrical-surface

The strong electronegativity of O dictates that the ground state of singlet CO has positively charged C and negatively charged O, in agreement with ab initio charge analysis, but in disagreement with the dipole direction. Though this unusual phenomenon has been fairly studied, the study of electrostatic potential (EP) for noncovalent interactions of CO is essential for better understanding. Here we illustrate that both C and O atom-ends show negative EP (where the C end gives more negative EP), favoring positively charged species, whereas the cylindrical surface of the CO bond shows positive EP, favoring negatively charged ones. This is demonstrated from the interactions of CO with Na+, Cl-, H2O, CO and benzene. It can be explained by the quadrupole driven electrostatic nature of CO (like N2) with very weak dipole moment. The EP is properly described by the tripole model taking into account the electrostatic multipole moments, which has a large negative charge at a certain distance protruded from C, a large positive charge on C, and a small negative charge on O. We also discuss the EP of the first excited triplet COopen0

Illicit and Counterfeit Drug Analysis by Morphologically Directed Raman Spectroscopy

Morphologically directed Raman spectroscopy (MDRS) is a novel tool for the forensic analysis of illicit and counterfeit drug samples. MDRS combines Raman microspectroscopy with automated particle imaging so that physical and chemical information about the components of a mixture sample can be obtained. Results of automated particle imaging are used to determine samples for Raman analysis. The use of MDRS for these types of samples can be employed for both forensic investigations and adjudications of cases. The method provides insight about the physical and chemical composition of the sample, as well as about manufacturing and sample history. Here, MDRS was used in four different illicit and counterfeit drug analyses: (1) examination of a multicomponent drug mixture where the results could be used for comparative source attribution, (2) the detection of low (or trace) concentration particles in a drug sample, (3) the analysis of synthetic cathinone samples (i.e., bath salts), and (4) a study of counterfeit pharmaceutical products

The dipolar endofullerene HF@C60

The cavity inside fullerenes provides a unique environment for the study of isolated atoms and molecules. We report encapsulation of hydrogen fluoride inside C60 using molecular surgery to give the endohedral fullerene HF@C60. The key synthetic step is the closure of the open fullerene cage while minimizing escape of HF. The encapsulated HF molecule moves freely inside the cage and exhibits quantization of its translational and rotational degrees of freedom, as revealed by inelastic neutron scattering and infrared spectroscopy. The rotational and vibrational constants of the encapsulated HF molecules were found to be redshifted relative to free HF. The NMR spectra display a large 1H-19F J coupling typical of an isolated species. The dipole moment of HF@C60 was estimated from the temperature-dependence of the dielectric constant at cryogenic temperatures and showed that the cage shields around 75% of the HF dipole

Fluorescence detected microwave Stark effect measurements in excited vibrational states of H2CO

We describe a microwave-optical double resonance experiment designed for Stark effect measurements on molecules in excited vibrational energy levels of ground electronic states. This technique, which employs pulsed laser excitation and laser induced fluorescence detection, has been used to measure electric dipole moments in the 1(1), 5(1), and 5(2) vibrational states of formaldehyde. In addition, a Stark induced quantum beat experiment is briefly described. (C) 2003 American Institute of Physics

Photodissociation detection of microwave transitions in highly excited vibrational states

This paper describes a new spectroscopic technique which uses photodissociation to detect pure rotational transitions in highly excited vibrational energy levels of the ground electronic state. This method has initially been applied to the (HOCl)-Cl-35 molecule in its second excited OH stretching vibration, the (200) state. In addition to observing the microwave signal, the Stark effect for this transition provides a dipole moment measurement in the (200) vibrational state. The (200) 18(0,18) --> 17(1,17) transition occurs at 14 858.40(1) MHz and the b component of the dipole moment of this state is mu(b) = 1.515(5) D. The technique is quite general in that the rotational transition can involve energy levels up to, and exceeding, the dissociation threshold of the ground electronic state. (C) 1999 American Institute of Physics. [S0021-9606(99)00632-7]

The dipole moment of HOCl in v(OH)=4

Pulsed laser excitation and photofragment detection methods are used to observe the 17(0,17) <-- 16(1,16) pure rotational transition within the v(OH) = 4 vibrational state of (HOCl)-Cl-35. Microwave frequency and Stark effect measurements give v(0) = 27484.33(10) MHz and mu(b) = 1.562(9) D. The dependence of mu(b), which is approximately parallel to the OH bond, on the level of OH stretch excitation appears linear and is consistent with that of H2O over the same 0-14000 cm(-1) energy range. (C) 2003 Elsevier Science (USA). All rights reserved

Hyperfine and tunneling effects in the microwave spectrum of N-2-OCS

A- and b-type rotational transitions of N2-OCS between 6 and 18 GHz have been recorded using a pulsed, supersonic nozzle, Fourier-transform microwave spectrometer. The rotational spectrum is indicative of a planar, T-shaped complex, with the nitrogen molecular axis oriented toward the carbon of OCS. Symmetry effects in the nuclear quadrupole hyperfine structure imply that the nitrogen nuclei are effectively equivalent due to rotation of the nitrogen molecule. The resulting symmetric and antisymmetric combined nuclear spin and tunneling states are associated with slightly different rotational and hyperfine parameters. First-order quadrupole and spin-rotation interactions are used to fit the hyperfine structure, and the hyperfine dependence on rotational state is investigated. An instance of accidental near-degeneracy has allowed determination of the χab off-diagonal quadrupole coupling constant for the symmetric state. Deduced structural parameters have been compared with a model based on distributed multipole, dispersion, and hard sphere interactions. The quadrupole coupling constants of the two tunneling symmetry states have been used to model the angular tunneling potential, giving a barrier to rotation of 40.44 cm-1 and a tunneling frequency of 2450.0 GHz. © 1996 Academic Press, Inc

Dipole moments of highly vibrationally excited water

The intensity of water absorption in the region of the solar spectrum plays a dominant role in atmospheric energy balance and hence strongly influences climate. Significant controversy exists over how to model this absorption accurately. We report dipole moment measurements of highly vibrationally excited water, which provide stringent tests of intensities determined by other means. Our measurements and accompanying calculations suggest that the best currently available potential and dipole surfaces do not accurately model intensities in the optical spectrum of water