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

A proposed intramolecular hydrogen bond in the 2‐haloethanols has been investigated with analysis of the microwave spectra of 2‐chloroethanol and 2‐bromoethanol. For each moledule, only a form gauche about the CC bond has been observed. For the chloro compound nine isotopic species have been studied to yield the molecular coordinates of the chlorine, oxygen, hydroxyl hydrogen, and carbon atoms, and thus a molecular structure. For bromoethanol a molecular structure has been found based upon the coordinates of the bromine and hydroxyl hydrogen atoms. Principal structural conclusions are that, of the probable rotameric forms, the lowest energy form is the one allowing close approach of hydrogen and halogen—namely, the gauche–gauche for dihedral angles (CCX) (COH) close to 60°. The O–H bond length in chloroethanol is 1.008 A, about 5% longer than in ethanol. The H···X distance is ∼ 0.5 A less than the sum of the atom van der Waals radii for X = Cl, Br. The O···X distance is approximately equal to the sum of t...

Microwave Spectra and Intramolecular Hydrogen Bonding in the 2‐Haloethanols: Molecular Structure and Quadrupole Coupling Constants for 2‐Chloroethanol and 2‐Bromoethanol

Das Modell der Valenzelektronenpaarabstosung (VSEPR) – auch als Gillespie-Nyholm-Regeln bekannt – bietet seit vielen Jahren eine nutzliche Grundlage fur das Verstandnis und die Erklarung von Molekulgeometrien und wird wegen seiner Einfachheit als padagogisches Hilfsmittel breit akzeptiert. Die ursprungliche Formulierung des Modells basierte auf dem Konzept, das sich die Elektronenpaare in der Valenzschale gegenseitig abstosen und daher so weit wie moglich voneinander entfernt anordnen. In den letzten Jahren wurde das Augenmerk jedoch mehr auf den Raum gelegt, den die Valenzelektronenpaare einnehmen, d.h. auf die Elektronenpaardomanen sowie auf die relativen Grosen und Formen dieser Domanen. Diese neuformulierte Version des VSEPR-Modells ist einfacher anzuwenden und zeigt deutlicher, das das Pauli-Prinzip die physikalische Grundlage des Modells ist. Daruber hinaus zeigten Bader und seine Mitarbeiter durch die Analyse der Elektronendichteverteilung bei vielen kovalenten Molekulen, das es lokalisierte Anhaufungen von Elektronendichte (Ladungskonzentrationen) in den Valenzschalen der Atome eines Molekuls gibt, die die gleiche relative Lage und Grose haben, wie fur die Elektronenpaardomanen im VSEPR-Modell angenommen wurde, wodurch dieses Modell noch zusatzlich gestutzt wird. Dieses erweiterte Verstandnis des Elektronenpaarabstosungs-Modells loste Bemuhungen aus, die Elektronendichteverteilung in Molekulen, die lange als Ausnahmen der VSEPR-Theorie galten, zu analysieren, um diese Ausnahmen besser verstehen zu konnen. Es zeigte sich, das es oftmals wichtig ist, nicht nur die relativen Lagen und Grosen, sondern auch die Formen der Domanen von Bindungs- und freien Elektronenpaaren zur Erklarung der Details von Molekulgeometrien heranzuziehen. Ebenfalls wurde deutlich, das die grundlegende Annahme des VSEPR-Modells, die Rumpfelektronen eines Atoms, die unter der Valenzschale liegen, seien kugelformig verteilt und ubten keinen Einflus auf die Molekulgeometrie aus, in der Regel fur Nichtmetalle gultig ist, fur Metalle, einschlieslich der Ubergangsmetalle, jedoch oft nicht zutrifft. Die Verteilung der Rumpfelektronen eines polarisierbaren Metallatoms kann von der Kugelsymmetrie abweichen, da nichtbindende Elektronen enthalten sein konnen oder die Elektronenverteilung durch Liganden verzerrt sein kann; die Rumpfelektronen haben in solchen Fallen einen bedeutenden Einflus auf die Molekulgeometrie.

Elektronendomänen und das VSEPR-Modell der Molekülgeometrie

The isoelectronic analogs of the title molecules are screened for possible unknown species using ab initio HF or MP2 6-31G ∗ calculations. Some candidates are the bent cations OCNCO + and OCPCO + , the linear anions OBCBO 2− and OBNBO − , and the bent NBNBN 3− and NBOBN 2− . The known NC-NNN is predicted to have a CN 4 ( D 2d ) di(diazirine) isomer, only 2.7 eV above the cyanogen azide at HF level. The analogous BN 4 − is also found to have a local minimum. The predicted neutral species include the linear OCNBO and ONNBN and the bent OCPCN, rather resembling the experimentally known NCPCN − anion.

Ab initio studies of bonding trends: Part 9. The dicyanamide-carbon suboxide-dicyanoether-cyanogen azide isoelectronic series ABCDE1

The hidden kernel of molecular quasi-linearity: Quantum monodromy

The microwave spectra of S(CN)2, SC13CN2, SC2N15N, and 34S(CN)2 were studied in the frequency region 8 to 30 Gc/sec. Isotope shifts yield the following structural parameters: CS=1.701 A, ∠CSC=98°22′, CN=1.156 A, 2φ=108°24′, where 2φ is the angle between the two C–N internuclear lines. Thus the SCN angle deviates from linearity by 5°. From Stark‐effect measurements the dipole moment was determined to be 3.04±0.03 D. Quadrupole hyperfine structure partially resolved in several transitions of S(CN)2 and SC2N15N yields the following coupling constants (Mc/sec): S(CN)2 (χAA=−1.51, χBB=0.30, χCC=1.21) SC2N15N (χAA=−1.51, χBB=0.24, χCC=1.27). Centrifugal distortion constants and the inertial defect for a low‐lying vibrationally excited state (v4=1) of S(CN)2 were of great value in making the following assignment of normal frequencies: ω1(A1) 2190 cm−1 (CN stretch),ω5(B1) 2180 cm−1 (CN stretch),ω2(A1)  672 cm−1 (CS stretch),ω6(B1)  690 cm−1 (CS stretch),ω3(A1)  378 cm−1 (SCN def.),ω7(B1)  328 cm−1 (SCN def.),ω4(A...

Microwave and Infrared Spectra of Sulfur Dicyanide: Molecular Structure, Dipole Moment, Quadrupole Coupling Constants, Centrifugal Distortion Constants, and Vibrational Assignment

The microwave spectra of two isotopic species, CH3–CF2 35Cl and CH3–CF2 37Cl, have been investigated in the region 8–40 GHz. A partial determination of structure strongly suggests a shortening in C–C, C–F, and C–Cl distances compared to singly or doubly substituted fluoro‐ or chloroethanes. Vibrationally excited states were observed showing doublets due to the hindered internal rotation of the methyl group. An analysis of these doublets gives a barrier to internal rotation of 4400 ± 100 cal/mole. The chlorine quadrupole coupling tensor is nearly symmetric about the C–Cl bond. This implies a pitfall in the application of simple resonance theory to molecules of this type; with simple resonance theory the shortened C–Cl bond is explained in terms of considerable double‐bond character for the C–Cl bond, which in turn would call for an asymmetric χ tensor.

Microwave spectrum, barrier to internal rotation, structural information, and quadrupole coupling of 1-chloro-1, 1-difluoroethane.

The microwave spectra of the ground state and excited torsional states of several isotopic species of methane selenol have been observed. The barrier hindering internal torsion has been determined to be 1001±50 cal/mole. The structural parameters are C–Se= 1.959 A, Se–H= 1.473 A, C–H= 1.088 A, ∠C–Se–H= 95.45°, ∠H–C–H= 110.00°, with the symmetry axis of the methyl group tilted 1.5° away from the C–Se internuclear line. The dipole moment components are μA=1.253 D, μB=0.314 D, μtotal=1.291 D. The implications of this barrier height determination for theories on the orgin of the barrier are discussed.

Microwave spectrum, barrier to internal rotation, structure, and dipole moment of methane selenol

The microwave spectra of the ground state of the 35Cl and 37Cl isotopic species of 1‐chloro‐1‐fluoroethane have been investigated in the K‐band region. An analysis of the 35Cl quadrupole splittings shows that the χ tensor is asymmetric, presumably owing to the partial double bond character of the C–C1 bond. A satellite spectrum of the 35Cl isotope has been assigned to the first excited state of the torsion. The relative intensity of the satellite spectrum leads to a value of the barrier hindering internal torsion of 4300±300 cal/mole.

Microwave spectrum, quadrupole coupling, and barrier to internal rotation of 1‐chloro‐1‐fluoroethane

Microwave spectrum, quadrupole couling, and barrier to internal rotation of 1‐chloro‐1‐fluoroethane

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