High resolution millimeter-wave spectroscopy of vinyltellurol

The millimeter-wave rotational spectrum of vinyltellurol has been recorded and assigned for the first time. To support the spectrum assignment, high level ab initio calculations have been carried out. Geometries, total electronic energies, and harmonic vibrational frequencies have been determined at the MP2 level. A small-core relativistic pseudopotential basis set (cc-pVTZ-PP) was employed to describe the tellurium atom. Two stable conformers, synperiplanar (sp) and anticlinal (ac), have been identified. The sp conformer is planar with a small negative inertia defect of -0.025 u ?(2). The ac conformer was found to be nonplanar with a C-C-Te-H dihedral angle of about 140° from sp. This conformer exhibits a large amplitude motion associated with the torsion about the C-Te bond. The barrier to internal rotation is about 1 kJ/mol, according to the theoretical calculations. For the ac conformation, a torsional potential function consisting of quartic and quadratic terms of the torsional angle has been partially determined from the observed rotational constants.     

Structure of tetracarbonylethyleneosmium: ethylene structure changes upon complex formation

Rotational spectra of seven isotopomers of tetracarbonylethyleneosmium, Os(CO)4(eta2-C2H4), were measured in the 4-12 GHz range using a Flygare-Balle-type pulsed-beam Fourier transform microwave spectrometer system. Olefin-transition metal complexes of this type occur extensively in recent organic syntheses and serve as important models for transition states in the metal-mediated transformations of alkenes. Three osmium ((192)Os, (190)Os, and (188)Os) and three unique 13C isotopomers (13C in ethylene, axial, and equatorial positions) were observed in natural abundance. Additional spectra were measured for a perdeuterated sample, Os(CO)4(eta2-C2D4). The measured rotational constants for the main osmium isotopomer ((192)Os) are A = 929.3256(6), B = 755.1707(3), and C = 752.7446(3) MHz, indicating a near-prolate asymmetric top molecule. The approximately 140 assigned b-type transitions were fit using a Watson S-reduced Hamiltonian including A, B, C, and five centrifugal distortion constants. A near-complete r0 gas-phase structure has been determined from a least-squares structural fit using eight adjustable structural parameters to fit the 21 measured rotational constants. Changes in the structure of ethylene on coordination to Os(CO)4 are large and well-determined. For the complex, the experimental ethylene C-C bond length is 1.432(5) A, which falls between the free ethylene value of 1.3391(13) A and the ethane value of 1.534(2) A. The angle between the plane of the CH2 group and the extended ethylene C-C bond ( angleout-of-plane) is 26.0(3) degrees , indicating that this complex is better described as a metallacyclopropane than as a pi-bonded olefin-metal complex. The Os-C-C-H dihedral angle is 106.7(2) degrees , indicating that the ethylene carbon atoms have near sp3 character in the complex. Kraitchman analysis of the available rotational constants gave principal axis coordinates for the carbon and hydrogen atoms in excellent agreement with the least-squares fit results. The new results on this osmium complex are compared with earlier work on the similar complex, tetracarbonylethyleneiron (Fe(CO)4(eta2-C2H4)). The ethylene structural changes upon coordination to the metal are found to be larger for the ethylene-osmium complex than for the analogous ethylene-iron complex, consistent with the expected greater pi donation for the osmium atom.     

A microwave and quantum chemical study of the conformational properties of etheneselenocyanate (H(2)C=CHSeC[triple bond]N)

The structural and conformational properties of etheneselenocyanate (H2C=CHSeC[triple bond]N) have been explored by microwave spectroscopy and quantum chemical calculations performed at the MP2/aug-cc-pVTZ and B3LYP/aug-cc-pVTZ levels of theory. The spectra of two rotameric forms were assigned. The more stable form has a synperiplanar conformation, whereas the less stable form has an anticlinal conformation characterized by a C-C-Se-C dihedral angle of 163(3) degrees from the synperiplanar position (0 degrees). The synperiplanar form was found to be 4.5(4) kJ/mol more stable than the anticlinal form by relative intensity measurements performed on microwave transitions. The spectra of several isotopologues and two vibrationally excited states were assigned for the synperiplanar conformer. The anticlinal rotamer displays a complicated pattern of low-frequency vibrational states, which is assumed to reflect the existence of a small potential hump at the antiperiplanar (180 degrees) conformation. The predictions made in the MP2 and B3LYP calculations are in reasonably good agreement with the experimental results in some cases, whereas rather large differences are seen for other molecular properties.     

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.     

Terahertz vibration-rotation-tunneling spectroscopy of the ammonia dimer: characterization of an out of plane vibration

The terahertz vibration-rotation-tunneling (VRT) spectrum of the ammonia dimer (NH(3))(2) has been measured between ca. 78.5 and 91.9 cm(-1). The dipole-allowed transitions are separated into three groups that correspond to the 3-fold internal rotation of the NH(3) subunits. Transitions have been assigned for VRT states of the A-A (ortho-ortho) combinations of NH(3) monomer states. The spectrum is further complicated by strong Coriolis interactions. K = 0 <-- 0, K = 1 <-- 0, K = 0 <-- 1, and K = 1 <-- 1 progressions have been assigned. The band origins, rotational constants, asymmetry doubling, centrifugal distortion, and Coriolis coupling constant have been determined from the fit to an effective Hamiltonian. These VRT transitions are tentatively assigned to an out of plane vibration with a K = 0 state at 89.141305(47) cm(-1), and a K = 1 state at 86.77785(9) cm(-1).     

Unraveling the spectroscopy of coupled intramolecular tunneling modes: a study of double proton transfer in the formic-acetic acid complex

The rotational spectrum of the hetero dimer comprising doubly hydrogen-bonded formic acid and acetic acid has been recorded between 4 and 18 GHz using a pulsed-nozzle Fourier transform microwave spectrometer. Each rigid-molecule rotational transition is split into four as a result of two concurrently ongoing tunneling motions, one being proton transfer between the two acid molecules, and the other the torsion/rotation of the methyl group within the acetyl part. We present a full assignment of the spectrum J = 1 to J = 6 for the ground vibronic states. The transitions are fitted to within a few kilohertz of the observed frequencies using a molecule-fixed effective rotational Hamiltonian for the separate A and E vibrational species of the G(12) permutation-inversion symmetry group. Interpretation of the motion problem uses an internal-vibration and overall-rotation angular momentum coupling scheme and full sets of rotational and centrifugal distortion constants are determined. The tunneling frequencies of the proton-transfer motion are measured for the ground A and E methyl rotation states as 250.4442(12) and -136.1673(30) MHz, respectively. The slight deviation of the latter tunneling frequency from being one half of the former, as simple theory otherwise predicts, is due to different degrees of mixing in wavefunctions between the ground and excited states.     

Fourier transform microwave spectrum of CO-dimethyl ether

Two sets of 32 rotational transitions were observed for the carbon monoxide-dimethyl ether (CO-DME) complex and two sets of 30 transitions for both (13)CO-DME and C(18)O-DME, in the frequency region from 3.5 to 25.2 GHz, with J ranging from 1<--0 up to 7<--6, by using a Fourier transform microwave spectrometer. The splittings between the two sets of the same transition varied from 2 to 15 MHz, and the two components were assigned to the two lowest states of the internal rotation of CO with respect to DME governed by a twofold potential. A preliminary analysis carried out separately for the two sets of the observed transition frequencies by using an ordinary asymmetric-rotor Hamiltonian indicated that the heavy-atom skeleton of the complex was essentially planar, as evidenced by the "pseudoinertial defects," i.e., the inertial defects, which involve the contributions of the out-of-plane hydrogens of the two methyl groups, I(cc)-I(aa)-I(bb) of -5.764(23) and -5.753(16) uA(2) for the symmetric and antisymmetric states, respectively. All of the observed transition frequencies were subsequently analyzed simultaneously, by using a phenomenological Hamiltonian which was described in a previous paper on Ar-DME and Ne-DME [Morita et al., J. Chem. Phys. 124, 094301 (2006)]. The rotational constants thus derived were analyzed to give the distance between the centers of gravity of the two component molecules, DME and CO, to be 3.682 A and the angle between the CO and the a-inertial axes to be 75.7 degrees ; the C end of the CO being closer to the DME. Most a-type transitions were observed as closely spaced triplets, which were ascribed to the internal rotation of the two methyl tops of DME. The V(3) potential barrier was obtained to be 772(2) cm(-1) from the first-order Coriolis coupling term between the internal rotation and overall rotation, which is about 82% of V(3) for the DME monomer, whereas the second-order contribution of the coupling to the B rotational constant led to V(3) of 705(3) cm(-1). By assuming a Lennard-Jones-type potential, the dissociation energy was estimated to be E(B)=1.6 kJ mol(-1), to be compared with 1.0 and 2.5 kJ mol(-1) for Ne-DME and Ar-DME, respectively.     

Microwave spectrum of propionyl chloride

The microwave spectrum of propionyl chloride has been investigated in the region 18.0–40.0 GHz, and transitions due to a cis conformer have been assigned. This form has a heavy atom planar configuration and the methyl group and the carbonyl oxygen atom are cis to each other. Using the substitution structures of propionic acid and acetyl chloride as molecular models for the propionyl chloride molecule, good agreement is found between observed and calculateò effective rotational constants. For the ³⁵Cl species satellite spectra assigned to the first four excited states of the C-C torsional mode have been observed together with the first excited state of the methyl torsional mode. The ground state spectrum has also been assigned for the ³⁷Cl species. Relative intensity measurements yielded the lowest C-C torsional vibration frequency of 86 ± 10 cm^?1. The CH₃ internal rotation frequency was found to be 197 cm^?1. Nuclear quadrupole coupling constants were determined for the ground state of the ³⁵Cl and ³⁷Cl species. From observed A-E splittings of ^bQ-branch transitions of the first excited state of the methyl torsional mode a barrier to internal rotation was estimated to be V₃ = 2480 ± 40 cal mol^?1 (867 ± 14 cm^?1).     

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.     

Determination of structural parameters for ferrocenecarboxaldehyde using Fourier transform microwave spectroscopy

Gas-phase structural parameters for ferrocenecarboxaldehyde have been determined using Fourier transform microwave spectroscopy. Rotational transitions due to a-, b-, and c-type dipole moments were measured. Eighteen rotational constants were determined by fitting the measured transitions of various isotopomers using a rigid rotor Hamiltonian with centrifugal distortion constants. Least-squares fit and Kraitchman analyses have been used to determine the gas-phase structural parameters and the atomic coordinates of the molecule using the rotational constants for various isotopomers. Structural parameters determined from the least-squares fit are the Fe-C bond lengths to the cyclopentadienyl rings, r(Fe-C)=2.047(4) A, and the distance between the carbon atoms of the cyclopentadienyl rings, r(C-C)=1.430(2) A and r(C1-C1')=1.46(1) A of ring carbon and aldehyde carbon atom. Structural parameters were also obtained using density-functional theory calculations, and these were quite helpful in resolving ambiguities in the structural fit analysis, and providing some fixed parameters for the structural analysis. The results of the least squares and the calculations indicate that the carbon atoms of the Cp groups for ferrocenecarboxaldehyde are in an eclipsed conformation in the ground vibrational state.     

Microwave spectroscopy measurements of rotational spectra and DFT calculations for two distinct structural isomers of 1,1'-dimethylferrocene

Microwave spectra were obtained for two distinct structural isomers of 1,1'-dimethylferrocene, an eclipsed synperiplanar isomer (phi = 0 degrees, the E0 isomer), with A = 1176.9003(2) MHz, B = 898.3343(2) MHz, C = 668.7469(2) MHz, and an eclipsed synclinal isomer (phi = 72 degrees, the E72 isomer) with A = 1208.7117(14) MHz, B = 806.4101(12) MHz, and C = 718.7179(8) MHz. The b-dipole, asymmetric-top spectra of both structural isomers were measured in the frequency range of 5-12 GHz using a Flygare-Balle type of spectrometer. A very good fit to observed transitions, with small distortion constants, was obtained for the E0 conformer, indicating that this conformer is nearly rigid. The deviations obtained in a similar least-squares fit for the E72 confomer are significantly larger, indicating possible fluxional behavior for this conformer. In addition, 7 out of the 26 transitions observed for the E72 isomer conformer clearly exhibit very small splittings, giving further evidence for internal motion. DFT calculations for the different possible conformations of 1,1'- dimethylferrocene arising from rotation of one methyl cyclopentadienyl ligand relative to the other about the nominal C5 axis by an angle phi (dihedral angle) were performed using the B3PW91 functional. The calculations converged and were optimized for five structures on this torsional potential energy surface corresponding to different dihedral angles phi; three yielded energy minima, and two gave energy maxima, corresponding to transition states. The experimental results are in very good agreement with the results of the DFT calculations.     

Microwave survey of the conformational landscape exhibited by the propeller molecule triethyl amine

Conformational studies with quantum chemical methods yielded for the most stable conformer of triethyl amine a propeller-like structure belonging to the point group C(3), which corresponds to an oblate top. The microwave spectrum of this conformer with (14)N hyperfine splitting of all rotational transitions was assigned and molecular parameters were determined. The rotational constants were found to be A = B = 2.314873978(11) GHz, the (14)N quadrupole coupling constant χ(cc) = -5.2444(07) MHz. The observed spectrum could be reproduced within experimental accuracy. The standard deviation of a global fit with 48 rotational transitions is 1.5 kHz. The propeller-like structure seems to be energetically favorable and therefore also typical for related systems like triethyl phosphine, triisopropyl amine, tri-n-propyl amine, and tri-tert-butyl amine. Furthermore, the rotational transitions of two isotopologues, (13)C(2) and (13)C(5), could be measured in natural abundance and fitted with an excellent standard deviation. The C rotational constants could be determined to be 1.32681(96) GHz and 1.32989(18) GHz for the (13)C(2) and (13)C(5) isotopologues, respectively.     

Microwave spectrum of ethylamine-gauche

The gauche rotamer of the ethylamine molecule was successfully assigned making ample use of MW-MW-DR-techniques. The transitions of the gauche form appear as quartets due to two large amplitude motions: the CN torsion and inversion. The rotational as well as interaction constants for all four states were fitted as well as their relative energies.     

High-resolution rovibrational analysis of the Nu4 band of cis-1,2-difluoroethylene

The diode laser spectrum of cis-1,2-CHF=CHF has been measured and analyzed in the nu4 fundamental region near 1016 cm(-1). This vibration of symmetry species A1 corresponds to the C-F symmetric stretching motion and gives rise to a strong b-type band. The rovibrational analysis, extended to the P, Q, and R branches, led to the identification of 2800 lines with J < or = 62, Ka < or = 18, Kc < or = 62. The assigned transitions free of major resonance contributions, fitted using Watson's A-reduction Hamiltonian in the Ir representation, yielded a set of spectroscopic parameters up to the quartic coefficients for the V4 = 1 state. Several perturbation effects occur throughout the band, mainly caused by the first-order c-type Coriolis interaction with the nu5 + nu11, vibrational state. Even though no transitions to the perturbing level were observed, the band orign and the rotational constants for the perturber were determined from a dyad model which includes the Coriolis interaction term.     

The gas phase structure of ethynylferrocene using microwave spectroscopy

Gas phase structural parameters for ethynylferrocene have been determined using microwave spectroscopy. Rotational transitions due to a- and b-type dipole moments were measured. Twenty four rotational constants have been determined by fitting the measured transitions of various isotopomers using a rigid rotor Hamiltonian with centrifugal distortion constants. Least-squares fits to determine structural parameters and Kraitchman analyses have been used to determine the gas phase structural parameters and the atomic coordinates from the rotational constants. The distance between the Fe atom and the C atoms of the cyclopentadienyl rings is r(Fe-C1) = 2.049(5) A, and the distance between the carbon atoms of the cyclopentadienyl ring is r(C-C) = 1.432(2) A. The ethynyl group is bent away from the Fe atom and out of the plane of the carbon atoms in the adjacent cyclopentadienyl ring by 2.75(6) degrees. Structural parameters were also obtained from DFT calculations and Kraitchman analyses, and the results are compared. Analysis of fit results for 13C isotopic substitution data indicates that the carbon atoms of the two cyclopentadienyl rings are in an eclipsed conformation in the ground vibrational state. Trends in microwave experimental values for the distance from the Fe atom to the center of the cyclopentadienyl ring for a series of substituted ferrocenes have been analyzed. This analysis provides an estimate of the gas phase distance from the Fe atom to the centers of the cyclopentadienyl rings for ferrocene of 1.65(1) A.     

The rotational spectrum of CuCCH(X̃ 1Σ+): a Fourier transform microwave discharge assisted laser ablation spectroscopy and millimeter/submillimeter study

The pure rotational spectrum of CuCCH in its ground electronic state (X? (1)Σ(+)) has been measured in the frequency range of 7-305 GHz using Fourier transform microwave (FTMW) and direct absorption millimeter/submillimeter methods. This work is the first spectroscopic study of CuCCH, a model system for copper acetylides. The molecule was synthesized using a new technique, discharge assisted laser ablation spectroscopy (DALAS). Four to five rotational transitions were measured for this species in six isotopologues ((63)CuCCH, (65)CuCCH, (63)Cu(13)CCH, (63)CuC(13)CH, (63)Cu(13)C(13)CH, and (63)CuCCD); hyperfine interactions arising from the copper nucleus were resolved, as well as smaller splittings in CuCCD due to deuterium quadrupole coupling. Five rotational transitions were also recorded in the millimeter region for (63)CuCCH and (65)CuCCH, using a Broida oven source. The combined FTMW and millimeter spectra were analyzed with an effective Hamiltonian, and rotational, electric quadrupole (Cu and D) and copper nuclear spin-rotation constants were determined. From the rotational constants, an r(m)(2) structure for CuCCH was established, with r(Cu-C) = 1.8177(6)?A?, r(C-C) = 1.2174(6)?A?, and r(C-H) = 1.046(2)?A?. The geometry suggests that CuCCH is primarily a covalent species with the copper atom singly bonded to the C≡C-H moiety. The copper quadrupole constant indicates that the bonding orbital of this atom may be sp hybridized. The DALAS technique promises to be fruitful in the study of other small, metal-containing molecules of chemical interest.     

Microwave spectra and barriers to internal rotation of z- and e-1-propenyl isocyanide

A synthetic procedure yielding a mixture of Z- and E-1-propenyl isocyanide (CH(3)CH═CHNC) is described. The microwave spectrum of this mixture has been recorded in the 12-100 GHz spectral range, and the spectra of the Z and E isomers have been assigned for the first time. Most transitions of the Z form were split into two components of equal intensity due to tunneling of the methyl group, which allowed the barrier to internal rotation of this group to be determined as 4.0124(12) kJ/mol by fitting 568 transitions with a maximum value of J = 46 using the computer program Xiam. This fit had a root-mean-square deviation as large as 4.325. The same transitions were therefore fitted anew using the more sophisticated program Erham. This fit has a rms deviation marginally better (4.136) than the Xiam fit. No split MW lines were found for E-1-propenyl isocyanide. The absence of splittings is ascribed to a barrier to internal rotation of the methyl group that is significantly higher than the barrier of the Z isomer. It is concluded that the barrier must be larger than 6 kJ/mol for the E form. The experimental work was augmented by quantum chemical calculations at CCSD/cc-pVTZ, B3LYP/cc-pVTZ, and MP2/cc-pVTZ levels of theory. The CCSD method predicts rotational constants of the Z and E forms well. The B3LYP barriers to internal rotation of a series of substituted propenes were calculated and found to be in good agreement with experiments. Calculations of the quartic centrifugal distortion constants of the two 1-propenyl isocyanides by the B3LYP and MP2 methods were less successful.     

High-resolution infrared spectrum of the nu1 Ba nd of eta5-C5H5NiNO

Gas-phase rotational constants and distortion constants have been determined for the nu1 (v=1) excited vibrational state of cyclopentadienylnickel nitrosyl (C5H5NiNO) using a high-resolution Fourier transform spectrometer system at Kitt Peak, Arizona. The rotationally resolved lines have been measured for the C-H symmetric stretch vibration (nu1=3110 cm(-1)). In the present analysis, over 150 lines have been assigned and fitted using a rigid-rotor Hamiltonian with centrifugal distortion. The vibrational band center, excited-state rotational constants, and distortion constants derived from the measured spectrum for this prolate symmetric-top molecule are nuo=3110.4129(4) cm(-1), A'=0.14328(8) cm(-1), B'=C'=0.041285(1) cm(-1), DJ'=0.078(1) kHz, DJK'=2.23(4) kHz, and DK'=-2.63(2) kHz, respectively. Several different combination differences, with a common upper state, were calculated for different K stacks for the observed spectra, and the consistency of the lower state rotational constants obtained provided further support for the current assignment. The ground-state rotational constant (B') derived from this combination differences analysis agrees with the previously obtained Fourier transform microwave value to within 0.15%. However, ground-state rotational constants, A' and B', have been fixed in the present analysis to avoid correlation effects and to get more accurate results. The new measured parameters are compared with the previously obtained results from Fourier transform microwave and infrared spectroscopy measurements. The C-H vibration stretching frequency and rotational constants were calculated using density functional theory calculations, and these were quite helpful in resolving ambiguities in the fitting procedure and for initial assignments of measured lines.     

The microwave spectrum,structure and centrifugal distortion constants of 2-chloroacrylonitrile

The microwave spectrum of 2-chloroacrylonitrile has been studied in the 26.5–40 GHz region. A total of 99 a- and b-type rotational transitions have been measured and assigned for CH₂ =C³⁵Cl(CN),yielding values for the rotational constants (in MHz): A = 6973.27, B = 3148.16, C = 2165.95. For CH₂=C³⁷Cl(CN) a total of 53 transitions have been measured and assigned and the rotational constants obtained are (in MHz): A = 6909.35, B = 3081.17, C = 2127.98. The distortion effects have also been studied and the quartic distortion constants have been evaluated. From the observed hyperfine structure, the chlorine nuclear quadrupole coupling constants have been obtained. The structure of vinyl cyanide and vinyl chloride can be transferred to account remarkably well for the observed rotational constants.     

Matrix isolation and low temperature solid state FTIR spectroscopic study of alpha-furil

Alpha-furil [C(4)H(3)O-C(=O)-C(=O)-C(4)H(3)O] has been isolated in argon and xenon matrices and studied by FTIR spectroscopy, supported by DFT(B3LYP)/6-311++G(d,p) calculations. The obtained spectra were fully assigned and revealed the presence in the matrices of three different conformers, all of them exhibiting skewed conformations around the intercarbonyl bond with the two C(4)H(3)O-C(=O) fragments nearly planar. The three conformers differ in the orientation of the furan rings relative to the carbonyl groups: the most stable conformer, I (C(2) symmetry; O=C-C=O intercarbonyl dihedral equal to 153.1 degrees), has both furan rings orientated in such a way that one of their beta-hydrogen atoms approaches the oxygen atom of the most distant carbonyl group, forming two H-C=C-C-C=O six-membered rings; the second most stable conformer, II (C(1) symmetry; O=C-C=O intercarbonyl dihedral equal to 126.9 degrees ), has one furan ring orientated as in I, while the second furan group is rotated by ca. 180 degrees (resulting in an energetically less favourable H-C=C-C=O five-membered ring); in the third conformer, III (C(2) symmetry; O=C-C=O dihedral equal to 106.2 degrees ), both furan rings assume the latter orientation relative to the dicarbonyl group. The theoretical calculations predicted the two higher energy forms being 5.85 and 6.22 kJ mol(-1) higher in energy than the most stable form, respectively, and energy barriers for conformational interconversion higher than 40 kJ mol(-1). These barriers are high enough to prevent observation of conformational isomerization for the matrix isolated compound. The three possible conformers of alpha-furil were also found to be present in CCl(4) solution, as well as in a low temperature neat amorphous phase of the compound prepared from fast condensation of its vapour onto a suitable 10 K cooled substrate. On the other hand, in agreement with the available X-ray data [S. C. Biswas, S. Ray and A. Podder, Chem. Phys. Lett., 1987, 134, 541], the IR spectra obtained for the neat low temperature crystalline state reveals that, in this phase, alpha-furil exists uniquely in its most stable conformational state, I.