Intra- and intermolecular H-bond mediated tautomerization and dimerization of 3-methyl-1,2-cyclopentanedione: Infrared spectroscopy in argon matrix and CCl 4 solution (original) (raw)
Related papers
Journal of Molecular …, 2011
Mid-infrared spectra of 3-methyl-1,2-cyclopentanedione (3-MeCPD) have been recorded by isolating the molecule in a cold argon matrix (8 K) and also in CCl 4 solution at room temperature. The spectral features reveal that in both media, the molecule exists exclusively in an enol tautomeric form, which is stabilized by an intramolecular OAHÁ Á ÁO hydrogen bond. NBO analysis shows that the preferred conformer is further stabilized because of hyperconjugation interaction between the methyl and vinyl group of the enol tautomer. In CCl 4 solution, the molecule undergoes extensive self association and generates a doubly hydrogen bonded centrosymmetric dimer. The dimerization constant (K d ) is estimated to have a value of $9 L mol À1 at room temperature (25°C) and the thermodynamic parameters, DH°, DS°and DG°, of dimerization are estimated by measuring K d at several temperatures within the range 22-60°C. The same dimer is also produced when the matrix is annealed at a higher temperature. In addition, a non-centrosymmetric singly hydrogen bonded dimer is also identified in the argon matrix. A comparison between the spectral features of the two dimers indicates that the dimerization effect on doubly H-bonded case is influenced by cooperative interaction between the two H-bonds.
The Journal of …, 2012
Molecular association and keto−enol tautomerization of β-cyclohexanedione (β-CHD) have been investigated in argon matrix and also in a thin solid film prepared by depositing pure β-CHD vapor on a cold (8 K) KBr window. Infrared spectra reveal that, in low-pressure vapor and argon matrix, the molecules are exclusively in diketo tautomeric form. The CH···O hydrogen bonded dimers of the diketo tautomer are produced by annealing the matrix at 28 K. No indication is found for keto−enol tautomerization of β-CHD in dimeric complexes in argon matrix within the temperature range of 8−28 K. On the other hand, in thin film of pure diketo tautomer, the conversion initiates only when the film is heated at temperatures above 165 K. The observed threshold appears to be associated with excitation of the intermolecular modes, and the IR spectra recorded at high temperatures display narrowing of vibrational bandwidths, which has been associated with reorientations of the molecules in the film. The nonoccurrence of tautomerization of the matrix isolated dimer is consistent with the barrier predicted by electronic structure calculations at B3LYP/6-311++G** and MP2/6-311++G** levels of theory. The transition state calculation predicts that CH···O interaction has a dramatic effect on lowering of the tautomerization barrier, from more than 60 kcal/mol for the bare molecule to ∼35−45 kcal/mol for dimers.
The Journal of Physical Chemistry A, 2012
Molecular association and keto−enol tautomerization of β-cyclohexanedione (β-CHD) have been investigated in argon matrix and also in a thin solid film prepared by depositing pure β-CHD vapor on a cold (8 K) KBr window. Infrared spectra reveal that, in low-pressure vapor and argon matrix, the molecules are exclusively in diketo tautomeric form. The CH···O hydrogen bonded dimers of the diketo tautomer are produced by annealing the matrix at 28 K. No indication is found for keto−enol tautomerization of β-CHD in dimeric complexes in argon matrix within the temperature range of 8−28 K. On the other hand, in thin film of pure diketo tautomer, the conversion initiates only when the film is heated at temperatures above 165 K. The observed threshold appears to be associated with excitation of the intermolecular modes, and the IR spectra recorded at high temperatures display narrowing of vibrational bandwidths, which has been associated with reorientations of the molecules in the film. The nonoccurrence of tautomerization of the matrix isolated dimer is consistent with the barrier predicted by electronic structure calculations at B3LYP/6-311++G** and MP2/6-311++G** levels of theory. The transition state calculation predicts that CH···O interaction has a dramatic effect on lowering of the tautomerization barrier, from more than 60 kcal/mol for the bare molecule to ∼35−45 kcal/mol for dimers.
Tautomeric properties and gas-phase structure of 3-methyl-2,4-pentanedione
Journal of Molecular Structure, 2012
The tautomeric and structural properties of 3-methyl-2,4-pentanedione, CH 3 AC(O)ACH(CH 3 )AC(O)ACH 3 , have been studied by gas-phase electron diffraction (GED) and quantum chemical calculations (B3LYP and MP2 approximation with different basis sets up to aug-cc-pVTZ). Analysis of GED intensities resulted in the presence of 100(3)% enol tautomer at 274(7)K. Quantum chemical calculations predict enol concentrations from 8% at MP2/6-31G(d,p) level to 100% at B3LYP/aug-cc-pVTZ. This would show the failure of the MP2/6-31G(d,p) method. The enol ring possesses C S symmetry with a strongly asymmetric hydrogen bond. The experimental geometric parameters are reproduced very closely by the B3LYP/aug-cc-pVTZ method.
Journal of Physical Chemistry A, 2010
Matrix-isolation infrared spectra of 1,2-cyclohexanedione (CD) and 3-methyl-1,2-cyclohexanedione (3-MeCD) were measured in a nitrogen matrix at 8 K. The spectral features reveal that, in the matrix environment, both molecules exist exclusively in the monohydroxy tautomeric form, which is stabilized by an intramolecular OsH · · · OdC hydrogen bond (HB). The ν OsH band of the enol tautomer of 3-MeCD appears at a relatively lower frequency and displays a somewhat broader bandwidth compared to that of CD, and these spectral differences between the two molecules are interpreted as being due to the formation of an interconnected CsH · · · O HB, where the enolic oxygen is the HB acceptor and one of the CsH covalent bonds of the methyl group is the HB donor. Electronic structure calculations at the B3LYP/6-311++G**, MP2/6-311++G**, and MP2/cc-pVTZ levels predict that this tautomer (enol-2) is ∼3.5 kcal/mol more stable than a second enolic form (enol-1) where such interconnected H-bonding is absent. Theoretical analysis with a series of molecules having similar functional groups reveals that part of the excess stability (∼1 kcal/mol) of enol-2 originates from a cooperative interaction between the interconnected CsH · · · O and OsH · · · O HBs. In the IR spectrum, a weak band at 3007 cm -1 is assigned to ν CsH of the methyl CsH bond involved in the H-bonded network. The spectra predicted by both harmonic and anharmonic calculations reveal that this transition is largely blue-shifted compared to the fundamentals of the other two methyl CsH stretching frequencies that are not involved in H-bonding. The conclusions are corroborated further by natural bond orbital (NBO) analysis.
Recent Advances in Spectroscopy, 2010
Matrix isolation infrared spectra of 1,2-cyclohexanedione (1,2-CHD) and 3-methyl-1,2-cyclohexanedione are measured in a nitrogen matrix at 8K temperature. The spectra reveal that in the matrix environment both the molecules exist exclusively in monohydroxy tautomeric forms with an intramolecular O-H· · · O=C hydrogen bonding. In the case of 3-MeCD, the fundamental of OH stretching νO−−H band appears more red-shifted with larger bandwidth indicating that the intramolecular O-H· · · O hydrogen bond of this molecule is somewhat stronger compared to that of 1,2-CD. Electronic structure calculations at B3LYP/6-311++G * * and MP2/cc-pVTZ levels predict that the monohydroxy tautomer of 1,2-CD is nearly 4.5 kcal/mol more stable than the corresponding diketo tautomer, but in the case of 3-MeCD, the stability difference between the diketo and preferred enol tautomer is more than 7.5 kcal/mol. Analysis of the geometric parameters reveals that the excess stabilization of the latter originates as a result of formation of an intramolecular O· · · H-O· · · H-C type interconnected hydrogen bonding network involving a methyl C-H bond, which interact in a cooperative fashion. The predicted infrared spectrum shows that the formation of such hydrogen bonding network causes large blue-shifting of the H-bonded methyl νC−−H transition, and this spectral prediction matches well with the features displayed in the measured spectrum.
The Journal of …, 2009
Matrix-isolation infrared spectra of 1,2-cyclohexanedione (CD) and 3-methyl-1,2-cyclohexanedione (3-MeCD) were measured in a nitrogen matrix at 8 K. The spectral features reveal that, in the matrix environment, both molecules exist exclusively in the monohydroxy tautomeric form, which is stabilized by an intramolecular OsH · · · OdC hydrogen bond (HB). The ν OsH band of the enol tautomer of 3-MeCD appears at a relatively lower frequency and displays a somewhat broader bandwidth compared to that of CD, and these spectral differences between the two molecules are interpreted as being due to the formation of an interconnected CsH · · · O HB, where the enolic oxygen is the HB acceptor and one of the CsH covalent bonds of the methyl group is the HB donor. Electronic structure calculations at the B3LYP/6-311++G**, MP2/6-311++G**, and MP2/cc-pVTZ levels predict that this tautomer (enol-2) is ∼3.5 kcal/mol more stable than a second enolic form (enol-1) where such interconnected H-bonding is absent. Theoretical analysis with a series of molecules having similar functional groups reveals that part of the excess stability (∼1 kcal/mol) of enol-2 originates from a cooperative interaction between the interconnected CsH · · · O and OsH · · · O HBs. In the IR spectrum, a weak band at 3007 cm -1 is assigned to ν CsH of the methyl CsH bond involved in the H-bonded network. The spectra predicted by both harmonic and anharmonic calculations reveal that this transition is largely blue-shifted compared to the fundamentals of the other two methyl CsH stretching frequencies that are not involved in H-bonding. The conclusions are corroborated further by natural bond orbital (NBO) analysis.
The Journal of Physical Chemistry a, 2007
The structure, preferred conformers and vibrational spectra of the pyrazolo-thiazole precursor of extended dipole diazafulvenium methide intermediates, dimethyl 2,2-dioxo-1H,3H-pyrazolo[1,5c][1,3]thiazole-6,7-dicarboxylate (DPTD) were investigated in low-temperature noble gas matrices (Ar, Xe), low temperature neat amorphous and crystalline phases and in KBr pellet (crystal and melted phases) by infrared spectroscopy, supported by quantum chemical calculations. Two types of conformers were observed spectroscopically in the matrices and in the neat amorphous solid resulting from fast condensation of the vapour of the compound onto the cold (20 K) substrate of the cryostat. These conformers correspond to the two pairs of nearly degenerated structures exhibiting skew/cis