Kraitchman's Equations Research Articles

Spectres de rotation de la molécule de chlorobenzène: II. Variétés isotopiques multisubstituées et structure géométrique précise

In order to complete the r s structure of chlorobenzene given in a preceding paper, a variety of isotopic species of this molecule were synthesized and their microwave spectra studied. This made twenty isotopic species available, enabling the determination of the geometrical parameters by a least squares method. Fitting only differences of moments of inertia either for monosubstituted species or to multiply substituted species gave the same result. They hardly differ from the r o values and agree with the r s values obtained by the Kraitchman equations. The resulting error limits were reduced, however. The following structural parameters were obtained, C 1C 2 = 1.399 Å, C 2C 3 = 1.386 Å, C 3C 4 = 1.3976 Å, C 1Cl = 1.7248 Å, C 2H 2 = 1.080 Å, C 3H 3 = 1.081 Å, C 4H 4 = 1.081 Å, C 6C 1C 2 = 120° 16, C 1C 2C 3 = 119°78, C 2C 3C 4 = 120°24, C 3C 4C 5 = 119°80, C 1C 2H 2 = 119°45, C 2C 3H 3 = 119°76. The structure of the ring differs significantly from C 6 symmetry. The deformation can be regarded as a compression of the position C 1 while the angle of C 2H 2 bond is also changed.

Spectra and structure op organogermanes: XVIII . Microwave spectrum of trimethylchlorogermane

The rotational spectra of four Ge and two Cl isotopic species of trimethylchlorogermane have been investigated in the region 18.0–40.0 GHz. The ground state rotational constants. B o, were found to be 1993.11, 1991.34, 1989.61, 1987.92, 1942.42, 1940.51 and 1938.62 MHz for the (CH 3) 3 70Ge 35Cl, (CH 3) 3 72Ge 35Cl, (CH 3) 3 74Ge 35Cl, (CH 3) 3 76Ge 35Cl, (CH 3) 3 70Ge 37Cl, (CH 3) 3 72Ge 37Cl and (CH 3) 3 74Ge 37Cl molecules, respectively. The utilization of Kraitchman's equations leads to an r s value of the GeCl distance of 2.170 ± 0.001 Å. With this value of the GeCl bond length and an assumed structure for the methyl group, the restraint equations gave the following two structural parameters: r GeC = 1.940 Å and ∠ CGeCl = 105.9°. A least-squares fit obtained by an iterative adjustment of the three heavy-atom structural parameters to the seven ground state rotational constants gave identical results within the experimental uncertainty. The centrifugal distortion constant, D j , was found to have a value of 5.6 ±1.0 × 10 −4 MHz. The determined structural parameters are compared with the corresponding ones for similar molecules and the longer Ge—Cl distance as compared with that in the H 3GeCl molecule is consistent with the electron donating ability of the methyl groups.

Microwave spectrum and structure of isoxazole

The investigation of the microwave spectrum of isoxazole ( ONCHCHC H) has been continued. The assignment of transitions up to J = 42 gave accurate rotational and distortion constants. The hyperfine splitting of the J = 0 → 1 transitions could be partially resolved and values for the quadrupole coupling constants were obtained. The use of microwave-microwave double resonance modulation in place of Stark effect modulation allowed the assignment of six vibrationally excited states and of the monosubstituted isotopic species containing 13C, 15N and 18O in their natural abundances. Kraitchman's equations for planar molecules were used to derive the r s-structure of the isoxazole ring. Application of a least squares technique for singular systems of normal equations gave structure parameters which optimally reproduce the observed changes of all three moments of inertia under isotopic substitution.

Microwave spectrum, structure and dipole moment of trimethylamine-borane

The ground state rotational spectra often isotopic species of trimethylamineborane, (CH 3) 3N 10BH 3, (CH 3) 3N 11BH 3, (CH 3) 3N 10BD 3, (CH 3) 3N 11BD 3, (CH 3) 3N 11BD 2H, (CD 3) 3N 10BH 3, (CD 3) 3N 11BH 3, (CD 3) 3N 10BD 3, (CD 3) 3N 11BD 3 and ( 13CH 3)( 12CH 3) 2N 11BH 3, have been measured and the effective moments of inertia obtained. The utilization of Kraitchman's equations leads to an r s value of the B-H distance of 1.211±0.003 Å and a NBH angle of 105.32±0.16°. By a least squares fit of the rotational constants the following structural parameters were obtained: r(NC) = 1.495 Å, r(BN) = 1.609 Å, and ∠BNC = 110.9°. The value of the dipole moment was found to be 4.59±0.13 D. A lower limit to the barrier to internal rotation of the BH 3 group was determined to be 3.4 kcal/mole.

On a Comparison of the Electron Diffraction and Microwave Spectroscopic Structures for t-Butyl Chloride

An rz structure for t-butyl chloride has been determined from previously published microwave rotational constants. The resulting parameters: rz (C–Cl) = 1.831 ± 0.015 Ao, rz(C–C) = 1.525 ± 0.005 Ao, rz (C–H) = 1.081 ± 0.004 Ao and >CCCl=107.0± 0.7° were found to be in excellent agreement with our recent electron diffraction investigation. It was found that small skeletal differences in the rz distances resulting from isotopic substitution substantially influenced the structure obtained for the parent molecule. An rAv structure was also obtained for the molecule by combining the microwave and electron diffraction data. A modified set of Kraitchman's equations for symmetric-top molecules is also given which includes corrections for isotopic changes in bond lengths. Equilibrium bond lengths for C–Cl(1.821±0.006 Å) and C–C(1.521±0.004 Å) estimated from the rAv structure are found to be substantially different from the r8 parameters for the corresponding bonds, rs(C–Cl)=1.803± 0.002 Å and rs(C–C)=1.530± 0.002 Å.

Microwave spectrum, dipole moment, and molecular structure of allyl mercaptan

The microwave spectra of normal and deuterated allyl mercaptan (CH 2CHCH 2SH and CH 2CHCH 2SD) were studied in the region from 8 to 33 gHz. About 160 lines have been assigned to the ground and the excited vibrational states of the “ gauche” form of the molecule. The rotational constants of the ground vibrational state of the normal species are A = 20041.68 mHz, B = 2795.72 mHz, C = 2701.10 mHz and those of deuterated species are A = 18540.30 mHz, B = 2774.35 mHz, C = 2662.71 mHz. The Stark effect measurements gave the components of the molecular dipole moment of the normal molecule as μ a = 1.24D, μ b = 0.21D, μ c = 0.42D, and total moment μ t = 1.33D; and μ a = 1.27D, μ b = 0.19D, μ c = 0.40D, and μ t = 1.34D for the deuterated species. Kraitchman's equations were used to calculate the position of the thiol (-SH) hydrogen atom.

Microwave Spectrum and Structure of Nitric Acid

The microwave spectra of five further isotopic species of nitric acid, H18ONO2, cis-HON18OO, trans-HONO18O, cis-HO15N18OO, and D15NO3, have been investigated. The data have been used to refine the previous structural determination of Millen and Morton. In the calculation of structure, particular consideration has been given to the use of ground-state moments of inertia in Kraitchman's equations for near-oblate planar molecules. The structure was calculated using several different methods, including the double-substitution method due to Pierce, in order to substantiate the following structural parameters: O–H=0.964, (H)O–N=1.406, cis–N–O=1.211, trans-N–O=1.199 Å, ∠HON=102°9′, ∠cis-ONO=115°53′, ∠trans-ONO=113°51′, and O···O (NO2 group)=2.184 Å. This determination clearly demonstrates a 2° tilt of the NO2 group away from the hydrogen atom. The NO2 parameters are close to those found in a number of related molecules, but there is some evidence that the cis-NO bond in HNO3 is slightly longer than the trans-NO bond. The NOH configuration is very similar to that found in formaldoxime. Further evidence for the exact planarity of nitric acid is given. Stark-effect measurements limit the out-of-plane component of the dipole moment μC to a value less than 1.1×10−3 D. The other components have been redetermined to have the values μA=1.99 D and μB=0.88 D, giving a total dipole moment μ=2.17±0.02 D.

Determination of structure by isotopic substitution in molecules with symmetrically equivalent atoms

Kraitchman's well-known equations relate changes in moments of inertia upon single isotopic substitution to the location of the substituted atom. These relations are here extended to the case of multiple isotopic substitution on a set of symmetrically equivalent atoms.

Microwave Spectrum, Structure, Dipole Moment, and Internal Barrier of Vinyl Silane

The structure of vinyl silane has been determined by studying the microwave spectra of each of its singly substituted isotopic species. Using Kraitchman's equations the following structural parameters were calculated: C=C1.347 ASi–C=C122∘53′ Si–H1.475H–Si–H108∘42′ C–H(cis)1.097C=C–H(cis)120∘18′C–H(trans)1.097 (assumed)C=C–H(trans)120∘38′C–H1.094C=C–H117∘59′Si–C1.853By studying the isotopic species containing an asymmetric silyl group the equilibrium conformation was found to be staggered, i.e., the silyl hydrogens are staggered with respect to the hydrogen on the central carbon. The symmetry axis of the silyl group was found to be tilted by an angle of 1°49′ from the Si–C bond axis towards the double bond. Comparison of the Si–C bond distance of vinyl silane with that of methyl silane indicates that recent estimates of the effect of changes in hybridization on the covalent radius of carbon are much too large. The barrier to internal rotation for vinyl silane has been calculated from splittings of groundstate rotational transitions of SiH3CHCH2 and splittings of first-excited state transitions of SiH3CHCH2 and SiD3CHCH2, and also from Stark effect measurements on the K = 1, J = 2←1 transitions of SiH3CHCH2. The various calculations are all in good agreement, and the best value for the barrier is found to be 1500±30 cal/mole. Quadratic Stark effect measurements on the J = 2←1 transitions gave μa = 0.648D, μb = 0.133D, and μ = 0.66D. The resultant dipole moment makes an angle of 11±2° with the a axis of SiH3CHCH2.