Molecular Mechanics (MM4) Studies of Carboxylic Acids, Esters, and Lactones

Lii, Jenn‐Huei

doi:10.1021/jp0142029

Cited by 24 publications

(33 citation statements)

References 61 publications

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“…As in the case of other carboxylic acids and esters, the cis (C) conformation is considerably more stable than the trans (T) arrangement. [48][49][50][51] The trans-cis (C-O) energy differences in MMAC are similar to those found previously for MCMAC, where the T conformers were disfavored relative to the most stable conformer by more than 30 kJ mol -1 . 35 For MMAC, the energies of the Tt and Tg conformers (relative to the most stable conformer (Ct)) were predicted by the calculations to be 35.5 and 46.4 kJ mol -1 .…”

Section: Resultssupporting

confidence: 77%

Methyl 3-Methyl-2H-azirine-2-carboxylate Photochemistry Studied by Matrix-isolation FTIR and DFT Calculations

et al. 2006

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The aliphatic 2H-azirine, methyl 3-methyl-2H-azirine-2-carboxylate (MMAC), has been synthesized and its monomeric form investigated by IR spectroscopy in an argon matrix, at 10 K, as well as theoretically (DFT/ B3LYP/6-311++G(d,p)). Two low-energy conformers of MMAC (Ct and Cc) were found in the matrix, both exhibiting the cis conformation around the C-O bond but differing in the arrangement around the C-C R bond. The two conformers were photoreactive upon in situ broadband UV excitation (λ > 235 nm), yielding nitrile ylide (P1) and ketene imine (P2) type products, which resulted from cleavage of the C-C or C-N bond, respectively. The kinetics of the reactions leading to the formation of P1 and P2 are of first order, with the processes being favored when the reactant is in the Cc conformation. Very interestingly, the C-N bond photocleavage, which is unusual for aliphatic 2H-azirines, was found to be preferred over the generally favored in 2H-azirines C-C bond breakage. This behavior is attributed to the presence in the molecule of the electronwithdrawing methoxycarbonyl substituent, which accelerates the intersystem crossing toward the T 1 triplet state and, in this way, favors the C-N bond cleavage. In addition to the primary photoprocesses leading to formation of P1 and P2, secondary photoprocesses leading to the decarboxylation and decarbonylation of P2 have been also observed.

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Section: Resultssupporting

confidence: 77%

Methyl 3-Methyl-2H-azirine-2-carboxylate Photochemistry Studied by Matrix-isolation FTIR and DFT Calculations

et al. 2006

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“…Indeed, the preference for the cis arrangement around the C-O bond has been found to constitute a very general rule for molecules bearing the carboxylic acid or carboxylic ester groups. [55][56][57][58] In MCMAC, the trans methoxycarbonyl conformation is further destabilized relative to the cis arrangement because in the former conformation strong steric repulsions between the methyl group and the chlorine atom exist. Accordingly, the OdC-C-Cl dihedral angle in both Tsk′ and Tsk is considerably different from 180°, contrary to what is observed in the two most stable conformers, where the OdC-C-Cl dihedral angle is close to 180°(see Figure 1).…”

Section: Resultsmentioning

confidence: 99%

Unusual Photochemical C−N Bond Cleavage in the Novel Methyl 2-Chloro-3-methyl-2H-azirine-2-carboxylate

et al. 2006

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The structure, preferred conformers, vibrational spectrum, and photochemical behavior of the novel azirine, methyl 2-chloro-3-methyl-2H-azirine-2-carboxylate (MCMAC) were investigated in low-temperature matrixes and in the neat solid amorphous state by infrared spectroscopy and quantum-chemical calculations. Two conformers of the compound were observed in argon, krypton, and xenon matrixes, in agreement with the DFT(B3LYP)/6-311++G(d,p) and MP2/6-311++G(d,p) theoretical calculations. Both conformers were found to exhibit the carboxylic ester group in the cis conformation, differing in the arrangement defined by the O=C-C-Cl dihedral angle (cis and trans, for Ct and Cc forms, respectively). The Ct conformer was found to be the most stable conformer in the gaseous phase as well as in both argon and krypton matrixes, whereas the more polar Cc conformer became the most stable form in the xenon matrix and in the neat solid amorphous phase. In situ broadband UV (lambda > 235 nm) excitation of matrix-isolated MCMAC led to azirine ring C-C and C-N bond cleavages, the latter process corresponding to the most efficient reaction channel. The photochemical cleavage of the C-N bond had never been previously observed in the case of aliphatic 2H-azirines. Two electron withdrawing substituents (methoxycarbonyl group and chlorine atom) are connected to the azirine ring in the novel MCMAC azirine. The simultaneous presence of these two groups accelerates intersystem crossing toward the triplet state where cleavage of the C-N bond takes place. The primary photoproducts resulting from the C-N and C-C ring-opening reactions were also found to undergo further photochemical decarbonylation or decarboxylation reactions.

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“…18,19 The trans to cis barrier associated with internal rotation around the C-O bond estimated by the same methods is 4300-4600 cm -1 . 19 These results are in good agreement with the data for formic and acetic acids (energy difference of ∼1400-1800 cm -1 and a trans to cis barrier of ∼3900-4400 cm -1 ). [25][26][27][28] For the matrix-isolated monomers of formic and acetic acids, the cis conformer is shortlived, even at 8 K, because of its conversion to the trans conformer by tunneling with a rate of ∼10 -2 -10 -3 s -1 , 3,7 which complicates the experimental characterization of the cis conformer of these molecules.…”

Section: Introductionmentioning

confidence: 90%

Internal Rotation in Propionic Acid: Near-Infrared-Induced Isomerization in Solid Argon

et al. 2005

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The conformational system of propionic acid (CH 3 CH 2 COOH) is studied in solid argon. It is predicted by the ab initio calculations that this molecule has four stable conformers. These four structures are denoted T t , T g ( , C t , and C g ( , and they differ by the arrangement around the C-O and C R -C bonds. The ground-state T t conformer is the only form present at 8 K after deposition of an argon matrix containing propionic acid. For the CH 3 CH 2 COOH and CH 3 CH 2 COOD isotopologues, narrow-band excitation of the first hydroxyl stretching overtone of the conformational ground state promotes the C R -C and C-O internal rotations producing the T g ( and C t conformers, respectively. A subsequent vibrational excitation of the produced T g ( form induces its conversion to the C g ( conformer by rotation around the C-O bond. In the dark, all of the produced conformers decay to the conformational ground state at different rates. The decay kinetics and its temperature dependence allow the identification of the conformers by IR absorption spectroscopy, which is supported by ab initio calculations of their vibrational spectra. For the CH 3 CH 2 COOD isotopologue, the excitation of molecules isolated in different matrix sites results in site-dependent photoisomerization rates for the C R -C and C-O internal rotations, which also confirm the identification of the photoproducts.

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Molecular Mechanics (MM4) Studies of Carboxylic Acids, Esters, and Lactones

Cited by 24 publications

References 61 publications

Methyl 3-Methyl-2H-azirine-2-carboxylate Photochemistry Studied by Matrix-isolation FTIR and DFT Calculations

Methyl 3-Methyl-2H-azirine-2-carboxylate Photochemistry Studied by Matrix-isolation FTIR and DFT Calculations

Unusual Photochemical C−N Bond Cleavage in the Novel Methyl 2-Chloro-3-methyl-2H-azirine-2-carboxylate

Internal Rotation in Propionic Acid: Near-Infrared-Induced Isomerization in Solid Argon

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