Rotational spectra and computational analysis of two conformers of leucinamide

Rotational spectra were recorded for two isotopic species of two conformers of the amide derivative of leucine in the range of 10.5-21 GHz and fit to a rigid rotor Hamiltonian. Ab initio calculations at the MP2/6-311++G(d,p) level identified the low energy conformations with different side chain configurations; the rotational spectra were assigned to the two lowest energy ab initio structures. We recorded 16 a- and b-type rotational transitions for conformer 1; the rotational constants of the normal species are A = 2275.6(2), B = 1033.37(2) and C = 911.71(5) MHz. We recorded 23 a- and b-type rotational transitions for conformer 2; the rotational constants of the normal species are A = 2752.775(8), B = 843.502(1) and C = 796.721(1) MHz. The rotational spectra of the (15)N(amide) isotopomer of each conformer were recorded and the atomic coordinates of the amide nitrogen were determined by Kraitchman's method of isotopic substitution. The experimentally observed structures are significantly different from the crystal structures of leucinamide and the gas-phase structures of leucine, and a natural bond orbital analysis revealed the donor-acceptor interactions governing side chain configuration.     

Rotational Spectra of 2,3,6-Trifluoropyridine: Effect of Fluorination on Ring Geometry

The ground state rotational spectrum of 2, 3, 6-trifluoropyridine has been investigated in the 2.0\begin{document}$ - $\end{document}20.0 GHz region by pulsed jet Fourier transform microwave spectroscopy. The experimental rotational constants are \begin{document}$ A $\end{document} = 3134.4479(2) MHz, \begin{document}$ B $\end{document} = 1346.79372(7) MHz, and \begin{document}$ C $\end{document} = 941.99495(6) MHz. The transitions are so intense that rotational transitions of all mono-\begin{document}$ ^{13} $\end{document}C and \begin{document}$ ^{15} $\end{document}N isotopologues are measured in natural abundance. The semi-experimental equilibrium rotational constants of the 7 isotopologues were derived by taking account of the anharmonic vibrational corrections, which allowed a semi-experimental determination of the equilibrium structure of 2, 3, 6-trifluoropyridine.     

Synthesis and characterization of (E)- and (Z)-3-mercapto-2-propenenitrile. Microwave spectrum of the Z-isomer

The kinetically unstable compound 3-mercapto-2-propenenitrile (HS-CH=CH-C[triple bond]N) has been prepared for the first time by flash vacuum pyrolysis at 800 degrees C of 3-(tert-butylthio)-2-propenenitrile with a yield of 77% and a Z:E ratio of 8:1. Several deuterium and 15N isotopologues were also prepared using isotopically enriched compounds. Quantum chemical calculations of the structural and conformational properties of the Z- and E-isomers were undertaken at the B3LYP/6-311++G(3df,2pd), MP2/6-311++G(3df,2pd), MP2/aug-cc-pVTZ, and G3 levels of theory. These methods all predict that the Z- and the E-forms each have two "stable" planar rotameric forms with the H-S-C=C link of atoms in either a synperiplanar or an antiperiplanar conformation, with the synperiplanar form of the Z-isomer as the global minimum. The Z-isomer has been investigated by means of Stark-modulation microwave spectroscopy. Spectra attributable to the parent and three deuterium-substituted isotopologues of a single conformer were recorded and assigned. Additionally, the spectrum belonging to the first excited state of the lowest bending vibration was assigned. The ground-state rotational constants obtained by the least-squares analysis of these transitions were found to be in excellent agreement with the corresponding approximate equilibrium values generated by the MP2/aug-cc-pVTZ calculations. The preferred conformer of this molecule was found to have a synperiplanar arrangement of the H-S-C=C chain of atoms and a planar or nearly planar geometry, with a stabilizing intramolecular hydrogen bond formed between the H atom of the thiol group and pi-electron density associated with the C[triple bond]N triple bond. The possible astrochemical/astrobiological significance of this compound is discussed.     

Synthesis, microwave spectrum, and dipole moment of allenylisocyanide (H2C═C═CHNC), a compound of potential astrochemical interest

An improved synthesis of a compound of potential astrochemical interest, allenylisocyanide (H(2)C═C═CHNC), is reported together with its microwave spectrum, which has been investigated in the 8-120 GHz spectral range to facilitate a potential identification in interstellar space. The spectra of the ground vibrational state and of five vibrationally excited states belonging to three different vibrational modes have been assigned for the parent species. A total of 658 transitions with a maximum value of J = 71 were assigned for the ground state and accurate values obtained for the rotational and quartic centrifugal distortion constants. The spectra of five heavy-atom ((13)C and (15)N) isotopologues were also assigned. The dipole moment was determined to be μ(a) = 11.93(16) × 10(-30) C m, μ(b) = 4.393(44) × 10(-30) C m, and μ(tot) = 12.71(16) × 10(-30) C m. The spectroscopic work has been augmented by theoretical calculations at the CCSD/cc-pVTZ and B3LYP/cc-pVTZ levels of theory. The theoretical calculations are generally in good agreement with the experimental results.     

Microwave spectra and gas phase structural parameters for N-hydroxypyridine-2(1H)-thione

The microwave spectrum for N-hydroxypyridine-2(1H)-thione (pyrithione) was measured in the frequency range 6-18 GHz, providing accurate rotational constants and nitrogen quadrupole coupling strengths for three isotopologues, C(5)H(4)(32)S(14)NOH, C(5)H(4)(32)S(14)NOD, and C(5)H(4)(34)S(14)NOH. Pyrithione was found to be in a higher concentration in the gas phase than the other tautomer, 2-mercaptopyridine-N-oxide (MPO). Microwave spectroscopy is best suited to determine which structure predominates in the gas phase. The measured rotational constants were used to accurately determine the coordinates of the substituted atoms and provided sufficient data to determine some of the important structural parameters for pyrithione, the only tautomer observed in the present work. The spectra were obtained using a pulsed-beam Fourier transform microwave spectrometer, with sufficient resolution to allow accurate measurements of the (14)N nuclear quadrupole hyperfine interactions. Ab initio calculations provided structural parameters and quadrupole coupling strengths that are in very good agreement with measured values. The experimental rotational constants for the parent compound are A = 3212.10(1), B = 1609.328(7), and C = 1072.208(6) MHz, yielding the inertial defect Δ(0) = -0.023 amu·?(2) for the C(5)H(4)(32)S(14)NOH isotopologue. The observed near zero inertial defect clearly indicates a planar structure. The least-squares fit structural analysis yielded the experimental bond lengths R(O-H) = 0.93(2) ?, R(C-S) = 1.66(2) ?, and angle (N-O-H) = 105(4)° for the ground state structure.     

Analysis of the rotational structure in the high-resolution infrared spectrum of trans-hexatriene-1-13C1: a semiexperimental equilibrium structure for the C6 backbone of trans-hexatriene

trans-Hexatriene-1-(13)C(1) (tHTE-1-(13)C(1)) has been synthesized, and its high-resolution (0.0015 cm(-1)) infrared spectrum has been recorded. The rotational structure in the C-type bands for ν(26) at 1011 cm(-1) and ν(30) at 894 cm(-1) has been analyzed. To the 1458 ground state combination differences from these bands, ground state rotational constants were fitted to a Watson-type Hamiltonian to give A(0) = 0.8728202(9), B(0) = 0.0435868(4), and C(0) = 0.0415314(2) cm(-1). Upper state rotational constants for the ν(30) band were also fitted. Predictions of the ground state rotational constants for tHTE-1-(13)C(1) from a B3LYP/cc-pVTZ model with scale factors based on the normal species were in excellent agreement with observations. Similar good agreement was found between predicted and observed ground state rotational constants for the three (13)C(1) isotopologues of cis-hexatriene, as determined from microwave spectroscopy. Equilibrium rotational constants for tHTE and its three (13)C(1) isotopologues, of which two were predicted, were used to find a semiexperimental equilibrium structure for the C(6) backbone of tHTE. This structure shows increased structural effects of π-electron delocalization in comparison with butadiene and some differences from the cis isomer of HTE. Structures predicted with the MP2/cc-pVTZ model are also compared.     

The gas-phase structure of alanine

The jet-cooled rotational spectrum of neutral alanine has been studied using laser-ablation molecular-beam Fourier transform microwave spectroscopy (LA-MB-FTMW). The spectra of the two most stable forms were observed in the frequency range 6-18 GHz for the parent, (15)N alanine, three single (13)C species, and four single D species. The (14)N nuclear quadrupole coupling hyperfine structures have been resolved, and their comparison with those calculated theoretically confirms unambiguously the conformer assignments. The independent structures of both conformers have been determined experimentally for the first time using r(s) and r(0) procedures. In both cases, the amino acid backbone is nonplanar. For the most stable conformer I, the COOH group adopts a cis configuration and an asymmetric bifurcated hydrogen bond is formed between the amino group and carbonyl oxygen (r(N-H(a)...O=C) = 2.70(2) A and r(N-H(b)...O=C) = 2.88(2) A). For conformer IIa, the COOH group adopts a trans configuration and is stabilized by a O-H...N hydrogen bond (r(O-H...N) = 1.96(2) A). The relative conformer abundances in the supersonic expansion have also been investigated.     

Microwave rotational spectra and structures of 2-fluoropyridine and 3-fluoropyridine

The ground state rotational spectra of 2-fluoropyridine and 3-fluoropyridine have been investigated using both Fourier transform microwave (FTMW) and chirped pulse Fourier transform microwave (cp-FTMW) spectroscopies. In addition to the parent species, the spectra of the (13)C and (15)N singly substituted isotopologues were recorded in the 8-23 GHz region in natural abundance. The rotational constants determined for the seven isotopologues of each were used to calculate relevant geometric parameters including the bond distances and angles of the pyridine ring backbone. The derived structures show a more pronounced deviation from the pyridine ring geometry when the fluorine substituent is ortho to nitrogen which is consistent with ab initio predictions at various levels of theory. Analysis of the (14)N hyperfine structure provided an additional source of information about the electronic structure surrounding the nitrogen atom as a function of fluorine substitution. Together, the experimental results are consistent with a bonding model that involves hyperconjugation whereby fluorine donates electron density from its lone pair into the π-system of pyridine.     

The Shape of the Simplest Non-proteinogenic Amino Acid α-Aminoisobutyric Acid (Aib)

The simplest non-proteinogenic amino acid α-aminoisobutyric acid (Aib), an analogue of glycine and alanine, has been vaporized by laser ablation and probed by high-resolution Fourier transform microwave spectroscopic techniques. Comparison of the experimental rotational and ¹⁴N nuclear quadrupole constants with that predicted ab initio has allowed the identification of three conformers of Aib exhibiting three types of hydrogen-bond interactions I (NH⋅⋅⋅O=C, cis-COOH), II (OH⋅⋅⋅N, trans-COOH), and III (N−H⋅⋅⋅O−H, cis-COOH) within the amino acid backbone. The observation of conformer III, not detected previously for related proteinogenic amino acids with a nonpolar side chain in a supersonic expansion, indicates that the presence of the methyl groups should restrict the conformational relaxation from conformer Aib-III to Aib-I. For conformer Aib-II, the rotational spectra of the ¹³C isotopomers reveal a tunneling motion arising from the two equivalent methyl groups in the molecule. The observation of a single spectrum at the midpoint between those predicted for the two ¹³C of the methyl groups has been explained by considering a double-minimum potential function with a low-energy interconversion barrier for a large amplitude internal motion. This singular fact has been corroborated by the anomalous centrifugal distortion effects determined in conformer Aib-II.     

How Water Interacts with Halogenated Anesthetics: The Rotational Spectrum of Isoflurane–Water

The rotational spectra of several isotopologues of the 1:1 complex between the inhaled anesthetic isoflurane and water have been recorded and analyzed by using Fourier transform microwave spectroscopy. The rotational spectrum showed a single rotamer, corresponding to the configuration in which the most stable conformer of isolated isoflurane is linked to the water molecule through an almost linear C?H???O weak hydrogen bond. All transitions display a hyperfine structure due to the ³⁵Cl (or ³⁷Cl) nuclear quadrupole effects.     

Microwave spectrum and conformational composition of 2-chloroethylisocyanide

2-Chloroethylisocyanide (ClCH(2)CH(2)N≡C) has been synthesized, and its microwave spectrum has been investigated in the 20-97 GHz spectral region. The spectra of (35)Cl and (37)Cl isotopologues of two conformers have been assigned. The Cl-C-C-N chain of atoms is antiperiplanar in one of these rotamers and synclinal in the second. The energy difference between the two forms has been obtained from relative intensity measurements. It was found that the antiperiplanar conformer is favored over the synclinal form by 4.3(8) kJ/mol. Quantum chemical calculations at the CCSD/cc-pVTZ and B3LYP/cc-pVTZ levels of theory have been performed. Most, but not all, of the spectroscopic constants predicted in these calculations are in good agreement with their experimental counterparts. The theoretical calculations correctly predict that the 2-chloroethylisocyanide exists as a mixture of an antiperiplanar and a synclinal conformer, with the former about 3.5 kJ/mol more stable than the latter. Both methods of calculations find that the antiperiplanar rotamer has a symmetry plane. The dihedral angle formed by the Cl-C-C-N link of atoms of the synclinal form is 67° according to the CCSD calculations. It is estimated from a comparison with the experimental rotational constants that this dihedral angle is uncertain by ±3°.     

Microwave measurements of proton tunneling and structural parameters for the propiolic acid-formic acid dimer

Microwave spectra of the propiolic acid-formic acid doubly hydrogen bonded complex were measured in the 1 GHz to 21 GHz range using four different Fourier transform spectrometers. Rotational spectra for seven isotopologues were obtained. For the parent isotopologue, a total of 138 a-dipole transitions and 28 b-dipole transitions were measured for which the a-dipole transitions exhibited splittings of a few MHz into pairs of lines and the b-type dipole transitions were split by ~580 MHz. The transitions assigned to this complex were fit to obtain rotational and distortion constants for both tunneling levels: A(0+) = 6005.289(8), B(0+) = 930.553(8), C(0+) = 803.9948(6) MHz, Δ(0+)(J) = 0.075(1), Δ(0+)(JK) = 0.71(1), and δ(0+)(j) = -0.010(1) kHz and A(0-) = 6005.275(8), B(0-) = 930.546(8), C(0-) = 803.9907(5) MHz, Δ(0-)(J) = 0.076(1), Δ(0-)(JK) = 0.70(2), and δ(0-)(j) = -0.008(1) kHz. Double resonance experiments were used on some transitions to verify assignments and to obtain splittings for cases when the b-dipole transitions were difficult to measure. The experimental difference in energy between the two tunneling states is 291.428(5) MHz for proton-proton exchange and 3.35(2) MHz for the deuterium-deuterium exchange. The vibration-rotation coupling constant between the two levels, F(ab), is 120.7(2) MHz for the proton-proton exchange. With one deuterium atom substituted in either of the hydrogen-bonding protons, the tunneling splittings were not observed for a-dipole transitions, supporting the assignment of the splitting to the concerted proton tunneling motion. The spectra were obtained using three Flygare-Balle type spectrometers and one chirped-pulse machine at the University of Virginia. Rotational constants and centrifugal distortion constants were obtained for HCOOH···HOOCCCH, H(13)COOH···HOOCCCH, HCOOD···HOOCCCH, HCOOH···DOOCCCH, HCOOD···DOOCCCH, DCOOH···HOOCCCH, and DCOOD···HOOCCCH. High-level ab initio calculations provided initial rotational constants for the complex, structural parameters, and some details of the proton tunneling potential energy surface. A least squares fit to the isotopic data reveals a planar structure that is slightly asymmetric in the OH distances. The formic OH···O propiolic hydrogen bond length is 1.8 ? and the propiolic OH···O formic hydrogen bond length is 1.6 ?, for the equilibrium configuration. The magnitude of the dipole moment was experimentally determined to be 1.95(3) × 10(-30) C m (0.584(8) D) for the 0(+) states and 1.92(5) × 10(-30) C m (0.576(14) D) for the 0(-) states.     

Chirped pulse Fourier transform microwave study of 2,2,2-trifluoroethyl formate

The rotational spectra of 2,2,2-trifluoroethyl formate and its three (13)C isotopologues have been measured with a molecular-beam-based, chirped-pulsed Fourier transform microwave spectrometer in combination with a conventional Balle-Flygare-type instrument up to 18 GHz. Although ab initio calculations predict the presence of two low-energy conformers (analogous to the trans and gauche forms of ethyl formate), the trans isomer was the only stable conformer observed. The r(s) geometry of the molecular main carbon frame was precisely derived based on a coplanar heavy-atom backbone of this conformer. ESPs of the two lowest energy conformers were calculated to obtain information about the role of through-space effects on their structures and relative stability.     

Conformational identification and dipole moment of 4-fluoro-1-butene by microwave spectroscopy

The microwave spectrum of 4-fluoro-1-butene has been recorded in the frequency region 26.5–40.0 GHz. A-type rotational transitions in the ground vibrational state have been assigned. The effective rotational constants and centrifugal distortion constants were determined to be A = 19 196 ± 323, B = 2132.48 ± 0.01, C = 2112.52 ± 0.01 MHz, D_J = 0.70 ± 0.03 and D_JK = −26.16 ± 0.05 kHz. Analysis of the measured Stark effects gave a dipole moment of 1.80 D with components of |u_a| = 1.62 ± 0.01, |u_b| = 0.68 ± 0.05 and |u_c| = 0.39 ± 0.14 D. From the experimental rotational constants and dipole moments, it is suggested that the assigned spectra have resulted from the skew—trans conformer.     

Microwave Spectrum and Structure of 2-(Trifluoromethyl)pyridine

The high resolution rotational spectrum of 2-(trifluoromethyl)pyridine in 2\begin{document}$ - $\end{document}20 GHz was recorded and analyzed. Spectroscopic parameters including rotational constants, nuclear quadrupole coupling constants of \begin{document}$ ^{14} $\end{document}N as well as the centrifugal distortion constants were determined. The rotational spectra of five mono-substituted \begin{document}$ ^{13} $\end{document}C and one \begin{document}$ ^{15} $\end{document}N isotopologues were also measured and assigned in natural abundance. Experimental results complemented by ab initio calculations lead to an accurate determination of the skeleton structure. The values of the planar moment inertia \begin{document}$ P_{cc} $\end{document} were determined to be 44.46 u?\begin{document}$ ^2 $\end{document} for all the measured isotopologues, indicating a C\begin{document}$ _ \rm{s} $\end{document} symmetry of this molecule. The molecular electrostatic surface potential was calculated to illustrate the trifluoromethyl substitution effects on the electron distribution.     

Millimeter-wave rotational spectroscopy of FeCN (X 4Δi) and FeNC (X 6Δi): determining the lowest energy isomer

The pure rotational spectrum of FeCN has been recorded in the frequency range 140-500 GHz using millimeter/sub-millimeter direct absorption techniques. The species was created in an ac discharge of Fe(CO)(5) and cyanogen. Spectra of the (13)C, (54)Fe, and (57)Fe isotopologues were also measured, confirming the linear cyanide structure of this free radical. Lines originating from several Renner-Teller components in the ν(2) bending mode were also observed. Based on the observed spin-orbit pattern, the ground state of FeCN is (4)Δ(i), with small lambda-doubling splittings apparent in the Ω = 5/2, 3/2, and 1/2 components. In addition, a much weaker spectrum of the lowest spin-orbit component of FeNC, Ω = 9/2, was recorded; these data are consistent with the rotational parameters of previous optical studies. The data for FeCN were fit with a Hund's case (a) Hamiltonian and rotational, spin-orbit, spin-spin, and lambda-doubling parameters were determined. Rotational constants were also established from a case (c) analysis for the other isotopologues, excited vibronic states, and for FeNC. The r(0) bond lengths of FeCN were determined to be r(Fe-C) = 1.924 ? and r(C-N) = 1.157 ?, in agreement with theoretical predictions for the (4)Δ(i) state. These measurements indicate that FeCN is the lower energy isomer and is more stable than FeNC by ~1.9 kcal/mol.     

Microwave spectrum and conformational composition of 2-fluoroethylisocyanide

The microwave spectrum of 2-fluoroethylisocyanide, FCH(2)CH(2)N≡C, has been investigated in the whole 50-120 GHz spectral region. Selected portions of the spectrum in the range of 18-50 GHz have also been recorded. The microwave spectra of the ground state and vibrationally excited states of two conformers have been assigned. Accurate spectroscopic constants have been derived from a large number of microwave transitions. The F-C-C-N chain of atoms is antiperiplanar in one of these rotamers and synclinal in the second conformer. The energy difference between the two forms was obtained from relative intensity measurements. It was found that the synclinal conformer is favored over the antiperiplanar form by 0.7(5) kJ/mol. Quantum chemical calculations at the high CCSD/cc-pVTZ and B3LYP/cc-pVTZ levels of theory were performed. Most, but not all, of the spectroscopic constants predicted in these calculations are in good agreement with the experimental counterparts. The theoretical calculations correctly indicate that the F-C-C-N dihedral angle in the synclinal form is about 67° but underestimate the magnitude of the gauche effect and erroneously predict the antiperiplanar rotamer to be 1.3-1.6 kJ/mol more stable than the synclinal conformer.     

The microwave spectrum of the 1,1-difluoroprop-2-ynyl radical, F2*C-C[triple bond]CH

The rotational spectrum of the 1,1-difluoroprop-2-ynyl radical, F2*C-C[triple bond]CH, a partially fluorinated variant of the propargyl radical, has been recorded in the ground electronic, 2B1, state using pulsed discharge, pulsed-jet, Fabry-Perot Fourier transform microwave spectroscopy. Five successive a-type rotational transitions, from N = 1-0 to N = 5-4, and Ka = 0, 1, and 2, were measured between 6.5 and 32.5 GHz with an uncertainty of 5 kHz. The molecular constants, including fine and hyperfine constants, were precisely determined. These constants are compared with our predictions based on a density functional theory level ab initio calculations and with the fine and hyperfine constants of the propargyl radical. The measured electron spin densities suggest that both the difluoropropargyl and the difluoroallenyl resonance forms [F2*C-C[triple bond]CH<-->F2C=C=C*H] make major contributions to the electronic structure of the radical.     

Chirped-pulse Fourier transform microwave spectroscopy of perfluoroiodoethane

The pure rotational spectrum of perfluoroiodoethane between 8.0 and 11.9 GHz has been measured on a search accelerated, correct intensity Fourier transform microwave (SACI-FTMW) spectrometer. The spectra is dense with 247 measured transitions in the given region. Only the anti conformer was observed for which rotational constants are reported. Nuclear electric quadrupole coupling constants due to the iodine-127 were determined and are reported. Also, two dipole forbidden/quadrupole allowed ΔJ=2 transitions were observed in the spectra. The observation of these transitions has been rationalized on the basis of near degeneracies between energy levels connected by χ_ab.     

The microwave and millimeter spectrum of ZnCCH (X̃2Σ+): a new zinc-containing free radical

The pure rotational spectrum of the ZnCCH (X?(2)Σ(+)) radical has been measured using Fourier transform microwave (FTMW) and millimeter direct-absorption methods in the frequency range of 7-260 GHz. This work is the first study of ZnCCH by any type of spectroscopic technique. In the FTMW system, the radical was synthesized in a mixture of zinc vapor and 0.05% acetylene in argon, using a discharge assisted laser ablation source. In the millimeter-wave spectrometer, the molecule was created from the reaction of zinc vapor, produced in a Broida-type oven, with pure acetylene in a dc discharge. Thirteen rotational transitions were recorded for the main species, (64)ZnCCH, and between 4 and 10 for the (66)ZnCCH, (68)ZnCCH, (64)ZnCCD, and (64)Zn(13)C(13)CH isotopologues. The fine structure doublets were observed in all the data, and in the FTMW spectra, hydrogen, deuterium, and carbon-13 hyperfine splittings were resolved. The data have been analyzed with a (2)Σ Hamiltonian, and rotational, spin-rotation, and H, D, and (13)C hyperfine parameters have been established for this radical. From the rotational constants, an r(m) ((1)) structure was determined with r(Zn-C) = 1.9083 A?, r(C-C) = 1.2313 A?, and r(C-H) = 1.0508 A?. The geometry suggests that ZnCCH is primarily a covalent species with the zinc atom singly bonded to the C≡C-H moiety. This result is consistent with the hyperfine parameters, which suggest that the unpaired electron is localized on the zinc nucleus. The spin-rotation constant indicates that an excited (2)Π state may exist ～19,000 cm(-1) in energy above the ground state.