Molecular Structure, Infrared Spectrum, and Photochemistry of Squaric Acid Dimethyl Ester in Solid Argon

Abstract

Squaric acid dimethyl ester (C(6)O(4)H(6); 3,4-dimethoxycyclobut-3-ene-1,2-dione; DCD) was studied by matrix isolation infrared spectroscopy and by density functional theory (B3LYP) and ab initio (MP2) calculations with the 6-31++G(d,p) and 6-311++G(d,p) basis sets. Three conformers of the compound were theoretically predicted. The two most stable conformers were identified in low-temperature argon matrixes and the energy gap between them was determined. The trans-trans conformer (C(2)(v)) was found to be more stable than the cis-trans form (C(s)) by 4.2 kJ mol(-1), in consonance with the theoretical predictions (MP2 calcd = 3.9 kJ mol(-1)). In situ broadband UV irradiation (lambda > 337 nm) of the matrix-isolated compound was found to induce the ring-opening reaction leading to production of the bisketene, 2,3-dimethoxybuta-1,3-diene-1,4-dione as well as the trans-trans --> cis-trans conformational isomerization. The latter phototransformation allowed separation of the infrared spectra of the two conformers initially trapped into a low-temperature matrix. Upon higher energy irradiation (lambda > 235 nm), the main observed photoproducts were CO and deltic acid dimethyl ester (C(5)O(3)H(6); 2,3-dimethoxycycloprop-2-en-1-one), the latter being obtained in two different conformations (trans-trans and cis-trans). According to the experimental data, deltic acid dimethyl ester is produced by decarbonylation of the initially formed bisketene and not by direct CO extrusion from DCD.

Conformational Space, IR-Induced, and UV-Induced Chemistry of Carvacrol Isolated in a Low-Temperature Argon Matrix

“…These photoreactions yield a molecule of CO and a complementary cyclic photoproduct with one carbon atom less in the cycle. Such cases of decarbonylation were reported for 2-indolinone, diglycolic anhydride, azacytosine, diketene, dimethyl ester of squaric acid, and isatin, to name a few. In the photochemistry of carvacrol, the decarbonylation could proceed via the initial hydroxy-oxo tautomerism, leading to an alkyl-substituted cyclohexadienone and subsequent formation of alkyl-substituted cyclopentadiene, exemplified as pathway 1 → [ 2 ] → 3b → 7b (see Scheme ).…”

Conformational changes in matrix-isolated 6-methoxyindole: Effects of the thermal and infrared light excitations

“…24,25,41 In all cases, no conformational relaxation was observed upon annealing the argon matrices up to the limit for the thermal stability of the host-material (∼35 K). Conformational conversions were only observed after depositing some of these molecules (i.e., those with accessible energy barriers for conformational isomerization) in xenon and heating the matrices up to temperatures ranging from 30 to 55 K (∼65 K is the limit for this host matrix).…”

“…21 On the other hand, in previous studies of molecules with conformers differing from each other by rotation of bulky fragments (such as OCH 3 or OCOH), no isomerizations have been observed after subjecting the matrices to broadband or narrowband IR radiation. [23][24][25] From a wider perspective, the results obtained in this work provide basic knowledge that can be used to understand local conformational features in many relevant methoxyindole-containing naturally occurring and synthetic biomolecules. [26][27][28][29][30] Also, the possibility of controlling the conformational distribution of 6MOI in matrices opens a window for exploiting its conformer-specific reactivity in condensed media (as opposite to the gas phase).…”

Tunable Diode Lasers as a Tool for Conformational Control: The Case of Matrix-Isolated Oxamic Acid

“…Prior to matrix deposition, the equilibrium ratio of conformers in the gas phase can be shifted by changing the temperature of the vapor. This approach is limited to compounds having conformers differing in energies by no more than 8 kJ mol –1 , so that they have non-negligible populations in the gas phase at moderate temperatures. − Several methods can be employed to manipulate relative populations of conformers already frozen in a low-temperature matrix. For instance, UV irradiation can induce alteration of the population ratio of conformers trapped in a matrix. − It was also observed for several species (such as glycolic acid , ) that excitation with broadband NIR/IR radiation (from the IR source of the spectrometer or from an external source) can lead to conformational changes.…”