Microwave spectrum and structure of the dimethylamine dimer: Evidence for a cyclic structure

Rotational spectra have been measured for six isotopomers of the dimethylamine dimer using a Fourier‐transform microwave spectrometer. No tunneling splittings were observed for either the a‐ or c‐type rotational transitions and the spectra could be fit to a rigid rotor Hamiltonian including centrifugal distortion terms. Possible dimer structures are discussed and compared to the experimentally determined moments of inertia, dipole moments, and nuclear quadrupole coupling constants. A cyclic structure with Cs symmetry was found to best reproduce the inertial data. The structure of the dimethylamine dimer is compared to the theoretical and experimental structures for the ammonia dimer.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70922/2/JCPSA6-100-5-3377-1.pd

The microwave spectrum, structure, and large amplitude motions of the methylacetylene⋅SO2 complex

Rotational spectra of five isotopomers of the methylacetylene⋅SO2 (MA⋅SO2) van der Waals complex have been observed with a Fourier transform microwave spectrometer. Each species showed two sets of rotational transitions, one associated with the A (m=0) and the other with the E (m=±1) methyl group internal rotation states. The rotational transitions of the isotopomers with S 16O2 and the doubly substituted S 18O2 also showed inversion splitting ranging from tens of kHz to a few MHz. This splitting was absent in the S 16O 18O isotopomers. The spectra of these species have been assigned and fit, yielding rotational constants, which allowed a complete determination of the structure of the complex. The SO2 was found to sit above the carbon–carbon triple bond, with one of the S–O bonds roughly parallel to the symmetry axis of methylacetylene. The centers‐of‐mass distance between the two monomers was determined to be 3.382(10) Å. The center frequencies of the inversion doublets (or quartets) were used in a fit of both the A and the E transitions; the barrier hindering the internal rotation of the methyl group was determined to be 62.8(5) cm−1. Based on the dependence of the inversion splitting on the transition dipole direction and isotopic substitution, the inversion motion was identified as an ‘‘in plane’’ wagging of the SO2 relative to methylacetylene. A pure inversion splitting of 3.11 MHz (free from rotation) was extracted from the A‐state spectrum of the normal species, from which an inversion barrier height of about 63 cm−1 was estimated.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69904/2/JCPSA6-101-8-6512-1.pd

Körner’s septum (petrosquamosal lamina): the anatomical variant or clinical problem?

Körner’s septum (KS) or petrosquamosal lamina is a bony lamina beginning at the articular fossa, extending above the middle ear, and running inferiorly and laterally to the facial nerve canal as it proceeds to the mastoid apex. This septum marks the junction of petrous and squamous bones. The paper presents details of the anatomical structure of KS, which is most often present at the level of the head of the malleus and/or the anterior semicircular canal. Attention is paid to embryological aspects of temporal bone development that lead to the formation of KS. Two imaging techniques most frequently used to diagnose KS are described, high resolution computed tomography (HRCT) and cone-beam computed tomography. Also presented is a case report of a 6-year-old patient suffering from chronic otitis media who developed a cholesteatoma due to presence of KS, illustrated with HRCT images and intraoperative capture. The authors describe diagnostic difficulties associated with this anatomical variant in the middle ear. The article also discusses the more frequent occurrence of this clinical problem in ears operated on due to chronic inflammation, retraction pocket or tympanosclerosis in comparison to healthy ears

The microwave spectrum, dipole moment and low frequency vibrational states for phosphabenzene

The microwave spectrum of the ground vibrational state and seven lowfrequency vibrational states for phosphabenzene (C5H5P) have been assigned. The small positive inertial defect and the alternation of intensities due to nuclear spin statistics for the ground state confirm a plannar structure with C2v symmetry. A PC bond distance of 1.70-1.73 A and 2 axis while one mode was symmetric. The dipole moment has been determined as 1.54+/-0.02 D.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/34207/1/0000496.pd

Structure of the chlorobenzene–argon dimer: Microwave spectrum and ab initio analysis

The rotational spectra of the 35Cl35Cl and 37Cl37Cl isotopes of the chlorobenzene–argon van der Waals dimer have been assigned using Fourier transform microwave spectroscopy techniques. Rotational constants and chlorine nuclear quadrupole coupling constants were determined which confirm that the complex has CsCs symmetry. The argon is over the aromatic ring, shifted from a position above the geometrical ring center towards the substituted carbon atom, and at a distance of about 3.68 Å from it. This distance is 0.1–0.2 Å shorter than the similar distance in the benzene–argon and fluorobenzene–argon complexes. Experimental results are confirmed and explained with the help of second-order Møller–Plesset perturbation calculations using a VDZP+diffVDZP+diff basis set. The complex binding energy of the chlorobenzene–argon complex is 1.28 kcal/mol (fluorobenzene–argon, 1.17; benzene–argon, 1.12 kcal/mol) reflecting an increase in stability caused by larger dispersion interactions when replacing one benzene H atom by F or by Cl. The structure and stability of Ar⋅C6H5–XAr⋅C6H5–X complexes are explained in terms of a balance between stabilizing dispersion and destabilizing exchange repulsion interactions between the monomers. © 2000 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70251/2/JCPSA6-113-20-9051-1.pd

Molecular structures of gas‐phase polyatomic molecules determined by spectroscopic methods

Spectroscopic data related to the structures of polyatomic molecules in the gas phase have been reviewed, critically evaluated, and compiled. All reported bond distances and angles have been classified as equilibrium (re), average (rz), substitution (rs), or effective (ro) parameters, and have been given a quality rating which is a measure of the parameter uncertainty. The surveyed literature includes work from all of the areas of gas‐phase spectroscopy from which precise quantitative structural information can be derived. Introductory material includes definitions of the various types of parameters and a description of the evaluation procedure.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87746/2/619_1.pd

The microwave spectrum, structure and nuclear quadrupole coupling constants for stibabenzene

The microwave spectrum of 121-SbC5H5, 123-SbC5H5, [beta]-dideutero 121-SbC5H3D2 and 123-SbC5H3D2 has been assigned in the region 26.5-40.0 GHz. The respective rotational constants and uncertainties are: A = 4512.69 +/- 0.42, B = 1738.00 +/- 0.01, C = 1254.51 +/- 0.01; A = 4512.84 +/- 0.30, B = 1729.80 +/- 0.01, C = 1250.22 +/- 0.01; A = 4176.18 +/- 0.33, B = 1660.94 +/- 0.01, C = 1188.15 +/- 0.01; A = 4176.60 +/- 0.61, B = 1652.94 +/- 0.03, C = 1184.03 +/- 0.03 (in MHz units). The structure is found to be planar, C2v in symmetry. The d(Sb-C) = 2.050 +/- 0.005 A and [angle]CSbC = 92.9[deg] +/- 1.0[deg]. The nuclear quadrupole coupling constants for the 121 and 123 antimony isotopes are [chi]aa = 456.4 +/- 4.1 MHz, [eta] = 0.396 +/- 0.008, and [chi]aa = 583.00 +/- 5.3 MHz, [eta] = 0.399 +/- 0.008, respectively. Several alternate techniques using the coupling constants as data support a [sigma]-donating property for antimony.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/22722/1/0000277.pd

The mechanism of propylene oxidation to acrolein over bismuth molybdate, copper oxide, and rhodium catalysts

The oxidation of (E)-propene-1-d1 to acrolein over bismuth molybdate, copper oxide, and rhodium catalysts was studied to determine if the reaction proceeded with (Z)-(E) randomization of the deuterium stereochemistry. Over Bi2Mo2O9 and Bi2MoO6, (E)-acrolein-3-d1:(Z)-acrolein-3-d1: acrolein-1-d1 was 1:1:1 consistent with a [sigma]-allyl intermediate which rapidly converts between two equivalent forms (*CHD---CH=CH2 h CHD=CH---*CH2). With copper oxide, (E)-acrolein-3-d1: (Z)-acrolein-3-d1: acrolein-1-d1, was 1: 1: 1.6. This points to a discrimination isotope effect consistent with a [sigma]-allyl intermediate without interconversion of equivalent forms. Over a Rh/[alpha]-Al2O3 catalyst, (E)-acrolein-3-d1: (Z)-acrolein-3-d1: acrolein-1-d1 was 1: 0.93: 0.89. This is consistent with an allyl intermediate along with a second minor nonallylic pathway which does not equilibrate the terminal carbon atoms. One or both of these processes occurred with some retention of the (E)-deuterium stereochemistry in the acrolein-3-d1. Over an unsupported Rh catalyst, (E)-acrolein-3-d1: (Z)-acrolein-3-d1: acrolein-1-d1 was 1: 0.68: 0.85. Only the allylic pathway is evident and the reaction process occurs with incomplete randomization (76 +/- 10%) of the (E)-deuterium stereochemistry in the acrolein-3-d1.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/25151/1/0000587.pd

The microwave spectrum of argon-phosphorus trifluoride

The argon-PF3 complex has been prepared in a supersonic expansion of Ar (98%) and PF3 (2%). A Fourier-transform micro-wave spectrometer employing a Fabry-Perot cavity was used to assign 28 rotational transitions. The rotational constants (MHz) and distortion constants (kHz) were A = 7332.468(10), B = 1023.055(2), C = 952.564(2), DJ = 3.53(1), DJK = 60.4(1) and d1 = -0.240(7). The argon atom is 3.953 A (rc.m.) from the PF3 center of mass and rc.m. makes an angle of 70.3[deg] with the C3 axis of the PF3.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26836/1/0000396.pd