Conformational properties of trimethyl phosphate monomer

Vapour phase deposition of phosphonate-containing alumina thin films using dimethyl vinylphosphonate as precursor

Phosphorous-containing materials are used in a wide array of fields, from energy conversion and storage to heterogeneous catalysis and biomaterials. Among these materials, organic-inorganic metal phosphonate solids and thin films present an interesting option, due to their remarkable thermal and chemical stability. Yet, the synthesis of phosphonate hybrids by vapour phase thin film deposition techniques remains largely unexplored. In this work, we present successful deposition of phosphonate-containing films using dimethyl vinylphosphonate (DMVP) as a phosphonate precursor. Two processes have been studied, being a three-step process comprising alternating exposure to trimethylaluminum (TMA), water (H2O) and DMVP (ABC process), and a four-step process with an extra O3 step following the DMVP pulse (ABCD process). The O3 treatment is employed for in situ functionalisation of the adsorbed phosphonate precursor, transforming the vinyl group into a carboxylic acid end group. For both processes, good precursor saturation was found, with the ABCD process exhibiting a more stable growth per cycle (0.54-0.38 Å per cycle) in the investigated temperature range (100-250 °C). Phosphonate features were visible in FTIR spectra for both films, with the ABCD films also exhibiting a carboxylate signal. XPS showed a higher P incorporation in the ABCD films deposited at 250 °C, although still moderate (P/Al = 0.27), consistent with an alumina structure with phosphonate inclusions. The film stability upon immersion in water was tested, showing a slow oxidation over the course of a week. Finally, annealing experiments in air demonstrated stable films up to 400 °C.

Methoxyacetone revisited

Conformational space of methoxyacetone (MA) was studied at the MP2/6-311++G(d,p) and DFT(B3LYP)/6-311++G(d,p) levels of theory. Computations predict MA to adopt four conformations, resulting from internal rotations around the O=C-C-O (Trans, Cis) and C-C-O-C (trans, gauche) dihedral angles. The Tt (Trans-trans) conformer is the most stable. The computed energies of two gauche (Tg and Cg) conformers fall in the 3-8 kJ mol-1 range above Tt and should account for 1/3 of the room-temperature gas-phase equilibrium. The energy of Ct form is 11 kJ mol-1 above Tt, and its expected population is negligible (below 1 %). In our earlier work, MA monomers were isolated in cryogenic argon matrices and characterized by infrared spectroscopy. In the experiment, only the most stable Tt conformer was detected in the sample. Signatures of the other conformers were not detected, either in freshly deposited samples, or in samples subjected to different UV irradiations. We rationalize those observations in terms of computed barriers for intramolecular torsions, indicating occurrence of conformational cooling during deposition. The experimental infrared spectrum of the Tt form is now assigned with the aid of anharmonic DFT computations. Exposure of MA to UV irradiation in the 300-260 nm range led to photolysis, according to the Norrish type II mechanism, resulting in dimer between enol acetone and formaldehyde observed as a cage-confined intermediate photoproduct. The subsequent photolysis resulted in the formation of carbon monoxide as the dominating photoproduct, formed in the Norrish type I photoreaction. Mechanistic interpretation of this photo decarbonylation reaction is presented.

Probing the Thermochemistry Properties and Rate Kinetics of Trimethyl Phosphate (TMP): An H-Atom Abstraction (HAA) Reactions Perspective

Trimethyl Phosphate (TMP), an organophosphorus liquid compound, is valued for its versatile qualities and applications in various fields. In modern chemical research and industry, processes involving Trimethyl Phosphate are optimized for minimal negative environmental impact, and scientific advancement is driven by adherence to stringent regulations to provide sustainable solutions and resource preservation. Thermochemical insights enhance our understanding of monomer incorporation, initiation, and propagation energetics. This study comprehensively investigates the thermochemistry and rate kinetics that govern H-atom abstractions in TMP through advanced computational techniques. The theoretical framework encompasses methodologies for conducting conformer searches, exploring transition states, and performing energy calculations. This study calculates rate constants for eight H-atom abstraction reactions involving TMP with stable species, O2 (oxygen), H (hydrogen), and radicals [ȮH (hydroxyl), ĊH3 (methyl), CH3Ȯ (methoxy), HȮ2 (hydroperoxyl), ṄH2 (amino), and ĊN (cyano)], and further analogies are related to barrier heights. Bond dissociation energies are also determined, highlighting TMP's susceptibility to various reaction pathways. The discussion and findings elucidate the need for further experimental validation for practical applications of TMP in chemical synthesis, combustion, flame-retardant technologies, environmental processes, and pharmaceutical research.

Conformational energies of reference organic molecules: benchmarking of common efficient computational methods against coupled cluster theory

We selected 145 reference organic molecules that include model fragments used in computer-aided drug design. We calculated 158 conformational energies and barriers using force fields, with wide applicability in commercial and free softwares and extensive application on the calculation of conformational energies of organic molecules, e.g. the UFF and DREIDING force fields, the Allinger's force fields MM3-96, MM3-00, MM4-8, the MM2-91 clones MMX and MM+, the MMFF94 force field, MM4, ab initio Hartree-Fock (HF) theory with different basis sets, the standard density functional theory B3LYP, the second-order post-HF MP2 theory and the Domain-based Local Pair Natural Orbital Coupled Cluster DLPNO-CCSD(T) theory, with the latter used for accurate reference values. The data set of the organic molecules includes hydrocarbons, haloalkanes, conjugated compounds, and oxygen-, nitrogen-, phosphorus- and sulphur-containing compounds. We reviewed in detail the conformational aspects of these model organic molecules providing the current understanding of the steric and electronic factors that determine the stability of low energy conformers and the literature including previous experimental observations and calculated findings. While progress on the computer hardware allows the calculations of thousands of conformations for later use in drug design projects, this study is an update from previous classical studies that used, as reference values, experimental ones using a variety of methods and different environments. The lowest mean error against the DLPNO-CCSD(T) reference was calculated for MP2 (0.35 kcal mol-1), followed by B3LYP (0.69 kcal mol-1) and the HF theories (0.81-1.0 kcal mol-1). As regards the force fields, the lowest errors were observed for the Allinger's force fields MM3-00 (1.28 kcal mol-1), ΜΜ3-96 (1.40 kcal mol-1) and the Halgren's MMFF94 force field (1.30 kcal mol-1) and then for the MM2-91 clones MMX (1.77 kcal mol-1) and MM+ (2.01 kcal mol-1) and MM4 (2.05 kcal mol-1). The DREIDING (3.63 kcal mol-1) and UFF (3.77 kcal mol-1) force fields have the lowest performance. These model organic molecules we used are often present as fragments in drug-like molecules. The values calculated using DLPNO-CCSD(T) make up a valuable data set for further comparisons and for improved force field parameterization.

UV-induced -OCH3 rotamerization in a matrix-isolated methoxy-substituted ortho-hydroxyaryl Schiff base

A new methoxy-substituted ortho-hydroxyaryl Schiff base, 4-(3-methoxy-2-hydroxybenzylidene-amino) phenol was synthesized from 4-aminophenol and 2-hydroxy-3-methoxybenzaldehyde in methanol solution and characterized by ¹H-NMR, ¹³C-NMR and infrared spectroscopies and elemental analysis. The compound was isolated in a cryogenic (10 K) argon matrix, and the analysis of the infrared spectrum of the matrix-isolated compound revealed that it corresponds to the E-enol-imine isomeric form, with 3 different conformers being present in the matrix. These conformers share as common structural features the conformation of the free hydroxyl group (trans relatively to the para-substituent of the ring) and the presence of an OH^…N intramolecular H-bond involving the methoxy-substituted phenol ring and the azomethine bridge, while they differ in the orientation of the methoxy-substituent group. The structures and relative energies of the conformers of the molecule, and relevant barriers for their interconversion were obtained through quantum chemical calculations, which were also used to calculate the infrared spectra of the different forms. Calculations were also carried out for the higher-energy Z-enol-imine and keto-amine forms of the compound. Upon UV (230 nm) irradiation, -OCH₃ rotamerization was observed, leading to conversion of the lowest energy conformer, where the methoxy group is aligned with the plane of the ring, into the other two conformers initially present in the matrix, in which the OCH₃ group is out-of-the-plane of the ring. As for other phenolic compounds previously studied, spontaneous quantum mechanical tunneling conversion of the cis-OH conformers present in the gas-phase into the three observed conformers was found to take place during matrix deposition.

Structural Relevance of Intramolecular H-Bonding in Ortho-Hydroxyaryl Schiff Bases: The Case of 3-(5-bromo-2-hydroxybenzylideneamino) Phenol

A new Schiff base compound, 3-(5-bromo-2-hydroxybenzylideneamino)phenol (abbreviated as BHAP) was synthesized and characterized by ¹H- and ¹³C- nuclear magnetic resonance and infrared spectroscopies. DFT/B3LYP/6-311++G(d,p) calculations were undertaken in order to explore the conformational space of both the E- and Z- geometrical isomers of the enol-imine and keto-amine tautomers of the compound. Optimized geometries and relative energies were obtained, and it was shown that the most stable species is the E-enol-imine form, which may exist in four low-energy intramolecularly hydrogen-bonded forms (I, II, V, and VI) that are almost isoenergetic. These conformers were concluded to exist in the gas phase equilibrium with nearly equal populations. On the other hand, the infrared spectra of the compound isolated in a cryogenic argon matrix (10 K) are compatible with the presence in the matrix of only two of these conformers (conformers II and V), while conformers I and VI convert to these ones by quantum mechanical tunneling through the barrier associated with the rotation of the OH phenolic group around the C–O bond. The matrix isolation infrared spectrum was then assigned and interpreted with help of the DFT(B3LYP)/6-311++G(d,p) calculated infrared spectra for conformers II and V. In addition, natural bond orbital (NBO) analysis was performed on the most stable conformer of the experimentally relevant isomeric form (E-enol-imino conformer V) to shed light on details of its electronic structure. This investigation stresses the fundamental structural relevance of the O–H···N intramolecular H-bond in o-hydroxyaryl Schiff base compounds.

Structural, spectroscopic, and photochemical study of ethyl propiolate isolated in cryogenic argon and nitrogen matrices

Ethyl propiolate (HC ≡ CCOOCH₂CH₃, EP) was studied experimentally by infrared spectroscopy in argon and nitrogen cryomatrices (15 K) and by quantum chemical calculations (at the DFT(B3LYP) and MP2 levels of theory). Calculations predict the existence of four conformers: two low-energy conformers (I and II) possessing the carboxylic moiety in the cis configuration (O=C-O-C dihedral equal to ~0°) and two higher-energy trans forms (O=C-O-C dihedral equal to ~180°; III and IV). The conformation of the ethyl ester group within each pair of conformers is either anti (C-O-C-C equal to 180°; in conformers I and III) or gauche (C-O-C-C equal to ±86.6° in II, and ± 92.5° in IV). The two low-energy cis conformers (I and II) were predicted to differ in energy by less than 2.5 kJ mol^-1 and were shown to be present in the studied cryogenic matrices. Characteristic bands for each one of these conformers were identified in the infrared spectra of the matrix-isolated compound and assigned taking into account the results of normal coordinate analysis, which used the geometries and harmonic force constants obtained in the DFT calculations. The two trans conformers (III and IV) were estimated to be 17.5 kJ mol^-1 higher in energy than the conformational ground state (form I) and were not observed experimentally. The unimolecular photochemistry of matrix-isolated EP (in N₂ matrix) was also investigated. In situ irradiation with UV light (λ > 235 nm) leads mainly to decarbonylation of the compound, with generation of ethoxyethyne, which in a subsequent photoreaction generates ketene (plus ethene).

UV-induced radical formation and isomerization of 4-methoxyindole and 5-methoxyindole

Monomers of 4-methoxyindole and 5-methoxyindole trapped in low-temperature xenon matrices (15-16 K) were characterized by IR spectroscopy, in separate experiments. Each compound was shown to adopt the most stable 1H-tautomeric form. The photochemistry of the matrix-isolated compounds was then investigated by exciting the matrices with narrowband UV light with λ ≤ 305 nm. Two main photoproducts, similar for each compound, have been detected: (1) 4-methoxy- or 5-methoxy-indolyl radical, resulting from cleavage of the N-H bond; (2) 3H-tautomers (4-methoxy- or 5-methoxy-) with the released hydrogen atom reconnected at the C3 ring carbon atom. The presence of the two types of photoproducts in the UV-irradiated matrices was confirmed by comparison of their B3LYP/6-311++G(d,p) calculated IR spectra with the experimental spectra emerging upon the irradiations. The mechanism of the observed phototransformations was elucidated by Natural Bond Orbital and Natural Resonance Theory computations on the methoxy-substituted indolyl radicals resulting from the N-H bond cleavage. The highest natural atomic spin densities were predicted at the C3 and N1 positions of the indolyl ring, corresponding to a predominance of the resonance structures with the radical centres located at these two atoms. As a whole, the obtained experimental and theoretical data allowed establishing a general pattern for the photochemistry of methoxyindoles under matrix-isolation conditions.

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

Conformational changes induced thermally or upon infrared excitation of matrix-isolated 6-methoxyindole were investigated. Narrowband near-infrared excitation of the first overtone of the N-H stretching vibration of each one of the two identified conformers is found to induce a selective large-scale conversion of the pumped conformer into the other one. This easily controllable bidirectional process consists in the intramolecular reorientation of the methoxy group and allowed a full assignment of the infrared spectra of the two conformers. Matrices with different conformational compositions prepared by narrow-band irradiations were subsequently used to investigate the effects of both thermal and broadband infrared excitations on the conformational mixtures. Particular attention is given to the influence of the matrix medium (Ar vs. Xe) and conformational effects of exposition of the sample to the spectrometer light source during the measurements.

Conformational distortion of α-phenylethyl amine in cryogenic matrices – a matrix isolation VCD study

MI-VCD spectroscopy reveals conformational perturbations of a chiral amine due to matrix packing effects.

Conformational preferences of 3,4-dihydroxyphenylacetic acid (DOPAC)

The conformational space of 3,4-dihydroxyphenylacetic acid (DOPAC), an important dopamine metabolite, has been investigated by quantum chemical methods (B3LYP and MP2, with the 6-311++G(d,p) basis set) and matrix-isolation infrared spectroscopy. Detailed analysis of the calculated potential energy surfaces of the molecule led to identification of thirteen unique conformers, all of them showing the acetic acid side chain out of the aromatic ring plane by 60-95°. According to the calculated Gibbs energies, the five lowest energy conformers make up 99.7% of the conformational mixture at 298.15K, exhibiting individual populations falling between 16% and 24%. The main conformational trends of this molecule were interpreted on the grounds of a thorough analysis of the structural parameters and by the application of the Natural Bond Orbital theory. The role of the intramolecular interactions on the relative stability and structure of the conformers was also investigated. The infrared spectrum of DOPAC was registered after isolation of its monomers in argon and xenon matrices. Only one of DOPAC forms populated in the gas phase could be trapped in both matrix gases. This result is in agreement with the predicted low energy barriers for conformational isomerization and is also supported by annealing experiments. The spectra of matrix-isolated model compounds, phenylacetic acid and catechol, were studied under the same experimental conditions. These data were used as references and assisted in the interpretation of the results obtained for DOPAC.

Conformational space and photochemistry of tyramine isolated in argon and xenon cryomatrixes

The infrared spectra of tyramine monomers trapped in low-temperature argon and xenon matrixes were recorded. The presence of the flexible ethylamino side chain gives rise to a complex conformational surface that contains several minima of relatively low energies, some of them stabilized by a weak N-H···π hydrogen bond interaction between the amino group and the phenyl ring. The experimental infrared spectra confirm the presence of at least two stable conformers isolated in the matrixes. Annealing experiments performed on the xenon matrix revealed a change in the relative population of the experimentally relevant conformers upon isolation in this polarizable matrix, compared to the gas phase. The general interpretation of the spectra was based on harmonic and anharmonic quantum chemical calculations, undertaken at the DFT/B3LYP and MP2 levels of theory with the 6-311++G(d,p) basis set. The photochemical behavior of the matrix-isolated compound upon narrow-band UV irradiation was also investigated. Identification of ketene species in the spectra of the irradiated matrixes suggests the occurrence of a ring-opening reaction, which in the xenon matrix occurs concomitantly with the conformational isomerization of tyramine.

Tautomers and UV-induced photoisomerization of a strongly intramolecularly H-bonded aromatic azo-dye: 1-(cyclopropyl)diazo-2-naphthol

Aromatic azo compounds have a wide range of industrial applications as dyes in optical and color-changing materials and can also be exploited in the design of new photodynamic molecular systems. The azo derivative 1-(cyclopropyl)diazo-2-naphthol was isolated in low-temperature cryogenic matrices, and its molecular structure, tautomeric equilibrium, and photochemical transformations were characterized by infrared spectroscopy and theoretical calculations. Only azo-enol forms having the OH group involved in a strong intramolecular hydrogen bond, forming a six-membered ring with the azo group, were found experimentally. Irradiation with a narrowband source in the near-UV range generates different rotameric and tautomeric azo-enol and keto-hydrazone forms that can be interconverted at different irradiation wavelengths.

Effects of microhydration on the electronic properties of ortho-aminobenzoic acid

High-level density functional electronic structure calculations have been performed to analyze the effect of microsolvation with water on the electronic properties of ortho-aminobenzoic acid (o-Abz). The hydrogen-bonded interaction of the o-Abz molecule with one to three water molecules, o-Abz···(H2O)n (n = 1–3), has been considered in two different situations, once the solvent water molecules are placed close to the carboxyl (−COOH) group of o-Abz producing the o-Abz···[H2O]nCOOH complexes and when the water molecules are placed close to the amino (−NH2) group producing the o-Abz···[H2O]nNH2 clusters. Variation of the vibrational spectra and energetics upon hydrogen-bond formation are analyzed and compared with available experimental data. The effect of cooperativity is also analyzed. Overall, the hydrogen-bonded o-Abz···[H2O]nCOOH clusters are found to be more stable than the o-Abz···[H2O]nNH2 clusters.

Stepwise conformational cooling towards a single isomeric state in the four internal rotors system 1,2-butanediol

The present work explores the possibilities of the matrix isolation technique in the structural characterisation of highly flexible molecules. To date, most studies of this type were carried out on molecules with three or less internal degrees of freedom and a few (less than 10) possible conformations. The molecule of 1,2-butanediol has four conformationally relevant three-fold rotational axes, which can result in 81 possible conformations. A detailed theoretical study, at the MP2 and DFT(B3LYP) levels of theory with the 6-311 + + G(d,p) basis set, revealed that more than 20 conformers of 1,2-butanediol have relative energies in a 0-10 kJ mol(-1) range and contribute appreciably to the gas phase equilibrium at room temperature. This fact renders conformational studies of the system extremely difficult under normal conditions. However, the method of matrix isolation permits the reduction of the number of populated conformational states in the experiment at low temperature due to the effect known as conformational cooling: low energy barriers promote the relaxation of the higher energy local minima into more stable structures. As a result of massive conformational cooling occurring upon matrix deposition, only five conformers of 1,2-butanediol were retained in the samples at 10 K. These conformers were identified using a combination of FTIR spectroscopy and extensive theoretical calculations of vibrational spectra. Annealing of the matrices up to 50 K resulted in the extreme case of conformational cooling related with the depopulation of all conformers into the most stable unique structure. The observed transformations were rationalized in terms of barriers to intramolecular rotation.

A simple model for metal cation-phosphate interactions in nucleic acids in the gas phase: alkali metal cations and trimethyl phosphate

Threshold collision-induced dissociation techniques are employed to determine the bond dissociation energies (BDEs) of complexes of alkali metal cations to trimethyl phosphate, TMP. Endothermic loss of the intact TMP ligand is the only dissociation pathway observed for all complexes. Theoretical calculations at the B3LYP/6-31G* level of theory are used to determine the structures, vibrational frequencies, and rotational constants of neutral TMP and the M+(TMP) complexes. Theoretical BDEs are determined from single point energy calculations at the B3LYP/6-311+G(2d,2p) level using the B3LYP/6-31G* optimized geometries. The agreement between theory and experiment is reasonably good for all complexes except Li+(TMP). The absolute M+-(TMP) BDEs are found to decrease monotonically as the size of the alkali metal cation increases. No activated dissociation was observed for alkali metal cation binding to TMP. The binding of alkali metal cations to TMP is compared with that to acetone and methanol.

Photochemistry and vibrational spectra of matrix-isolated 5-ethoxy-1-phenyl-1H-tetrazole

A combined matrix isolation FT-IR and theoretical DFT(B3LYP)/6-311++G(d,p) study of the molecular structure and photochemistry of 5-ethoxy-1-phenyl-1H-tetrazole (5EPT) was performed. A new method of synthesis of the compound is described. Calculations show three minima, very close in energy and separated by low-energy barriers (less than 4 kJ mol-1), in the ground-state potential energy profile of the molecule. The method of matrix isolation enabled the reduction of the number of populated conformational states in the experiment at low temperature due to the effect known as conformational cooling. As a result, the spectrum of the as-deposited matrix of 5EPT closely matches that of the most stable conformer predicted theoretically, pointing to the existence of only this conformer in the low-temperature matrixes. In this structure, the dihedral angle between the two rings, phenyl and tetrazole, is ca. 30 degrees, whereas the ethyl group stays nearly in the plane of the tetrazole ring and is as far as possible from the phenyl group. In situ UV irradiation (lambda > 235 nm) of the matrix-isolated 5EPT induced unimolecular decomposition, which led mainly to production of ethylcyanate and phenylazide, this later compound further reacting to yield, as final product, 1-aza-1,2,4,6-cycloheptatetraene. Anti-aromatic 3-ethoxy-1-phenyl-1H-diazirene was also observed experimentally as minor photoproduct, resulting from direct extrusion of molecular nitrogen from 5EPT. This species has not been described before and is now characterized by infrared spectroscopy for the first time.

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.

Conformational Study of Monomeric 2,3-Butanediols by Matrix-Isolation Infrared Spectroscopy and DFT Calculations

The FT-IR spectra of two diastereomers of 2,3-butanediol, (R,S) and (S,S), isolated in low-temperature argon and xenon matrixes were studied, allowing the identification of two different conformers for each compound. These conformers were characterized by a +/-gauche arrangement around the O-C-C-O dihedral angle, thus enabling the establishment of a very weak intramolecular hydrogen bond of the O...H-O type. No other forms of these compounds were identified in matrixes, despite the fact that these four conformers had calculated relative energies from 0 to 5.1 kJ mol(-1) and were expected to be thermally populated from 50 to 6% in the gaseous phase of each compound. The nonobservation of additional conformers was explained in terms of low barriers to intramolecular rotation, resulting in the conformational relaxation of the compounds during deposition of the matrixes. The barriers to internal rotation of the OH groups were computed to be less than 4 kJ mol(-1) and are easily overcome in matrixes within the family of conformers with the same heavy atom backbone. The barriers for intramolecular rearrangement of the O-C-C-O dihedral angle in both diastereomers were calculated to range from 20 to 30 kJ mol(-1). Interconversions between the latter conformers were not observed in matrixes, even after annealing up to 65 K. Energy calculations, barriers, and calculated infrared spectra were carried out at the DFT(B3LYP)/6-311++G theory. Additional MP2/6-311++G calculations of energies and vibrational frequencies were performed on the most relevant conformers. Finally, independent estimations of the hydrogen-bond enthalpy in the studied molecules were also obtained based on theoretical structural data and from vibrational frequencies (using well-established empirical correlations). The obtained values for -DeltaH for both diastereomers of 2,3-butanediol amount to ca. 6-8 kJ mol(-1).