Relative and absolute bond dissociation energies of sodium cation-alcohol complexes determined using competitive collision-induced dissociation experiments

ManuscriptAbsolute (R1OH)Na+-(R2OH) and relative Na+-(ROH) bond dissociation energies are determined experimentally by competitive collision-induced dissociation of (R1OH)Na+(R2OH) complexes with xenon in a guided ion beam mass spectrometer. The alcohols examined include ethanol, 1-propanol, 2-propanol, n-butanol, iso-butanol, sec-butanol, and tert-butanol, which cover a range in Na+ affinities of only 11 kJ/mol. Dissociation cross sections for formation of Na+(R1OH) + R2OH and Na+(R2OH) + R1OH are simultaneously analyzed with a model that uses statistical theory to predict the energy dependent branching ratio. The cross section thresholds thus determined are interpreted to yield the 0 K (R1OH)Na+-(R2OH) bond dissociation energies and the relative 0K Na+-(ROH) binding affinities. The relative binding affinities are converted to absolute 0 K Na+-(ROH) binding energies by using the absolute bond energy for Na+-C2H5OH determined previously in our laboratory as an anchor value. Comparisons are made to previous experimental and theoretical Na+-(ROH) thermochemistry from several sources. The absolute (R1OH)Na+-(R2OH) bond dissociation energies were also calculated using quantum chemical theory at the MP2(full)/6-311+G(2d,2p)//MP2(full)/6-31G(d) level (corrected for zero-point energies and basis set superposition errors) and are generally in good agreement with the experimentally determined values

Observation of trans-ethanol and gauche-ethanol complexes with benzene using matrix isolation infrared spectroscopy

Ethanol can exist in two conformers, one in which the OH group is trans to the methyl group (trans-ethanol) and the other in which the OH group is gauche to the methyl group (gauche-ethanol). Matrix isolation infrared spectra of ethanol deposited in 20 K argon matrices display distinct infrared peaks that can be assigned to the trans-ethanol and gauche-ethanol conformers, particularly with the O-H stretching vibrations.\footnote{Barnes, A. J.Hallam, H. E. \emph{Trans. Faraday Soc.}, \textbf{1970}, \emph{66}, 1932-1940.} Given this, matrix isolation experiments were performed in which ethanol (\chem{C_2H_5OH}) and benzene (\chem{C_6H_6}) were co-deposited in argon matrices at 20 K in order to determine if conformer specific ethanol complexes with benzene could be observed in the infrared spectra. New infrared peaks that can be attributed to the trans-ethanol and gauche-ethanol complexes with benzene have been observed near the O-H stretching vibrations of ethanol. The initial identification of the new infrared peaks as being due to the ethanol-benzene complexes was established by performing a concentration study (1:200 to 1:1600 S/M ratios), by comparing the co-deposition spectra with the spectra of the individual monomers, by matrix annealing experiments (35 K), and by performing experiments using isotopically labeled ethanol (\chem{C_2D_5OD}) and benzene (\chem{C_6D_6}). Quantum chemical calculations were also performed for the \chem{C_2H_5OH}-\chem{C_6H_6} complexes using density functional theory (B3LYP) and ab initio (MP2) methods. Stable minima were found for the both the trans-ethanol and gauche-ethanol complexes with benzene at both levels of theory and were predicted to have similar interaction energies. Both complexes can be characterized as H-$\pi$ complexes, in which the ethanol is above the benzene ring with the hydroxyl hydrogen interacting with the $\pi$ cloud of the ring. The theoretical O-H stretching frequencies for the complexes were predicted to be shifted from the monomer frequencies and from each other and these results were used to make the conformer specific infrared peak assignments

EVIDENCE OF INTERNAL ROTATION IN THE O-H STRETCHING REGION OF THE 1:1 METHANOL-BENZENE COMPLEX IN AN ARGON MATRIX

Co-depositions of methanol (chem{CH_3OH}) and benzene (chem{C_6H_6}) in an argon matrix at 20 K result in the formation of a 1:1 methanol-benzene complex (chem{CH_3OH}-chem{C_6H_6}) as evidenced by the observation of distinct infrared bands attributable to the complex near the O-H, C-H, and C-O stretching fundamental vibrations of chem{CH_3OH} and the hydrogen out-of-plane bending fundamental vibration of chem{C_6H_6}. Co-deposition experiments were also performed using isotopically labeled methanol (chem{CD_3OD}) and benzene (chem{C_6D_6}) and the corresponding deuterated complexes were also observed. Based on ab initio and density functional theory calculations, the structure of the complex is thought to be an H-$pi$ complex in which the chem{CH_3OH} is above the chem{C_6H_6} ring with the OH hydrogen atom interacting with the $pi$ cloud of the ring. Close inspection of the O-H and O-D stretching peaks of the complexes reveals small, distinct satellite peaks that are approximately 3 � 4 wn lower than the primary peak. A series of experiments have been performed to ascertain the nature of the satellite peaks. These consist of co-depositions in which the concentrations of both monomers were varied over a large range (1:200 to 1:1600 S/M ratios), annealing experiments (20 K to 35 K), and lower temperature cycling experiments (20 K to 8 K). Based on the results of these experiments, it is concluded that the satellite peaks are due to rotational structure and not due to matrix site effects, higher aggregation or distinct complex geometries. Given the rigidity of a low temperature argon matrix, it is proposed that the rotational motion responsible for the satellite peaks is internal rotation within the methanol subunit of the complex rather than overall molecular rotation of the complex

CHARACTERIZATION OF A HYDROGEN PEROXIDE-BENZENE COMPLEX USING MATRIX ISOLATION INFRARED SPECTROSCOPY

Matrix isolation infrared spectroscopy was used to characterize a 1:1 complex of hydrogen peroxide (\chem{H_2O_2}) with benzene (\chem{C_6H_6}). Co-deposition experiments with \chem{H_2O_2} and \chem{C_6H_6} were performed at 20 K using argon as the matrix gas. New infrared peaks attributable to the \chem{H_2O_2}-\chem{C_6H_6} complex were observed near the O-H stretching vibrations and the OH bending vibrations of the \chem{H_2O_2} monomer and near the hydrogen out-of-plane bending vibration of the \chem{C_6H_6} monomer. The initial identification of the newly observed infrared peaks to those of a \chem{H_2O_2}-\chem{C_6H_6} complex was established by performing several concentration studies in which the sample-to-matrix ratios of the monomers were varied between 1:100 to 1:1600, by comparing the resulting co-deposition spectra with the spectra of the individual monomers, and by matrix annealing experiments (30 – 35 K). Co-deposition experiments using isotopically labeled hydrogen peroxide (\chem{D_2O_2} and \chem{HDO_2}) and benzene (\chem{C_6D_6}) in argon were also performed and the analogous peaks for the isotopically labelled complexes were observed. A series of co-deposition experiments with \chem{H_2O_2} and \chem{C_6H_6} was also performed using nitrogen as the matrix gas. Quantum chemical calculations were performed for the \chem{H_2O_2}-\chem{C_6H_6} complex at the MP2/aug-cc-pVDZ and M06-2X/aug-cc-pVDZ levels of theory in order to obtain optimized complex geometries and predicted vibrational frequencies of the complex, which were compared to the experimental infrared spectra

INFRARED SPECTRA OF THE 2,3-DIHYDROPYRROL-2-YL AND 2,3-DIHYDROPYRROL-3-YL RADICALS ISOLATED IN SOLID PARA-HYDROGEN

The reaction of hydrogen atoms (H) with pyrrole (\chem{C_4H_5N}) in solid $para$-hydrogen ($p$-\chem{H_2}) matrices at 3.2 K has been studied using infrared spectroscopy. The production of H atoms for reaction with \chem{C_4H_5N} was essentially a three step process. First, mixtures of \chem{C_4H_5N} and \chem{Cl_2} were co-deposited in $p$-\chem{H_2} at 3.2 K for several hours, then the matrix was irradiated with ultraviolet light at 365 nm to produce Cl atoms from the \chem{Cl_2}, and finally the matrix was irradiated with infrared light to induce the reaction of the Cl atoms with $p$-\chem{H_2} to produce HCl and H atoms. Upon infrared irradiation, a series of new lines appeared in the infrared spectrum, resulting from the products of the H + \chem{C_4H_5N} reaction. To determine the grouping of lines to distinct chemical species, secondary photolysis was performed using 533-nm and 455-nm light-emitting diodes. Based on the secondary photolysis, it was determined that the majority of the new lines belong to two distinct chemical species, designated as set A (3491.0, 2754.4, 1412.7, 1260.4, 1042.8, 963.2, 922.1, 673.6 \wn) and set B (3468.3, 2784.9, 1470.6, 1449.5, 136.3, 1266.5, 1151.1, 1098.0, 960.6, 949.5, 924.0, 860.8, 574.2 \wn). The most likely reactions to occur under the low temperature conditions in solid $p$-\chem{H_2} are the addition of the H atom to the nitrogen atom or the two carbon atoms of \chem{C_4H_5N} to produce the corresponding hydrogen atom addition radicals (H-\chem{C_4H_5N}). Quantum-chemical calculations were performed at the B3PW91/6-311++G(2d,2p) level for the three possible H-\chem{C_4H_5N} radicals in order to determine the relative energetics and the predicted vibrational spectra for each radical. The addition of the H atom to carbons 2 and 3 is predicted to be exothermic by 112.1 and 76.1 kJ/mol, respectively, while the addition of the H atom to the nitrogen is predicted to be endothermic by 67.8 kJ/mol. When the lines in set A and B are compared to the scaled harmonic and anharmonic vibrational spectra for all three possible radicals, the best agreement for set A is with the radical produced by the addition to carbon 3 (2,3-dihydropyrrol-2-yl radical) and the best agreement for set B is with the radical produced by addition to carbon 2 (2,3-dihydropyrrol-3-yl radical)

CHARACTERIZATION OF A CARBON DIOXIDE-HEXAFLOUROBENZENE COMPLEX USING MATRIX_x000d_ ISOLATION INFRARED SPECTROSCOPY

Matrix isolation infrared spectroscopy was used to characterize a 1:1 complex of carbon dioxide (chem{CO_2}) with hexaflourobenzene (chem{C_6F_6}). Co-deposition experiments with chem{CO_2} and chem{C_6F_6} were performed at 20 K using argon as the matrix gas. New infrared peaks attributable to the chem{CO_2}-chem{C_6F_6} complex were observed near the O-C-O antisymmetric stretching vibration of the chem{CO_2} monomer and near the C-F stretching vibration of the chem{C_6F_6} monomer. The initial identification of the newly observed infrared peaks to those of a chem{CO_2}-chem{C_6F_6} complex was established by performing several concentration studies in which the sample-to-matrix ratios of the monomers were varied between 1:100 to 1:1600, by comparing the resulting co-deposition spectra with the spectra of the individual monomers, and by matrix annealing experiments (30 – 35 K). Co-deposition experiments were also performed using isotopically labeled carbon dioxide ($^{13}$chem{CO_2}) and the analogous peaks for the $^{13}$chem{CO_2}-chem{C_6F_6} complex were observed. Quantum chemical calculations were performed for the chem{CO_2}-chem{C_6F_6} complex at the MP2/aug-cc-pVDZ level of theory in order to explore the intermolecular potential energy surface of the complex and to obtain optimized complex geometries and predicted vibrational frequencies of the complex. The calculations for the exploration of the potential energy surface involved rigid scans along the intermolecular distance and various angle coordinates for several general orientations of the two monomers. Based on these calculations, full geometry optimizations were then performed and two stable complex minima were found: one in which the chem{CO_2} is perpendicular and centered to the chem{C_6F_6} ring ($Delta$E$_{int}$ = -7.9 kJ/mol) and one in which the chem{CO_2} is parallel to the chem{C_6F_6} ring but displaced from the center ($Delta$E$_{int}$ = -6.0 kJ/mol). Comparing the predicted vibrational spectra for both complexes to the observed experimental spectra, particularly for the O-C-O antisymmetric stretching region, it is concluded that both structures are present in the solid argon matrices

CHARACTERIZATION OF A HYDROGEN PEROXIDE-BENZENE COMPLEX USING MATRIX ISOLATION INFRARED SPECTROSCOPY

Matrix isolation infrared spectroscopy was used to characterize a 1:1 complex of hydrogen peroxide (\chem{H_2O_2}) with benzene (\chem{C_6H_6}). Co-deposition experiments with \chem{H_2O_2} and \chem{C_6H_6} were performed at 20 K using argon as the matrix gas. New infrared peaks attributable to the \chem{H_2O_2}-\chem{C_6H_6} complex were observed near the O-H stretching vibrations and the OH bending vibrations of the \chem{H_2O_2} monomer and near the hydrogen out-of-plane bending vibration of the \chem{C_6H_6} monomer. The initial identification of the newly observed infrared peaks to those of a \chem{H_2O_2}-\chem{C_6H_6} complex was established by performing several concentration studies in which the sample-to-matrix ratios of the monomers were varied between 1:100 to 1:1600, by comparing the resulting co-deposition spectra with the spectra of the individual monomers, and by matrix annealing experiments (30 – 35 K). Co-deposition experiments were also performed using isotopically labeled hydrogen peroxide (\chem{D_2O_2} and \chem{HDO_2}) and benzene (\chem{C_6D_6}) and the analogous peaks for the isotopically labelled complexes were observed. Quantum chemical calculations were performed for the \chem{H_2O_2}-\chem{C_6H_6} complex at the MP2/aug-cc-pVDZ level of theory in order to explore the intermolecular potential energy surface of the complex and to obtain optimized complex geometries and predicted vibrational frequencies of the complex, which were compared to the experimental infrared spectra

INFRARED SPECTRA OF THE 1-CHLOROMETHYL-1-METHYLALLYL AND 1-CHLOROMETHYL-2-METHYLALLYL RADICALS ISOLATED IN SOLID PARA-HYDROGEN

The reaction of chlorine atoms (Cl) with isoprene (chem{C_5H_8}) in solid $para$-hydrogen ($p$-chem{H_2}) matrices at 3.2 K has been studied using infrared spectroscopy. Mixtures of chem{C_5H_8} and chem{Cl_2} were co-deposited in $p$-chem{H_2} at 3.2 K, followed by irradiation at 365 nm to cause the photodissociation of chem{Cl_2} and the subsequent reaction of Cl atoms with chem{C_5H_8}. Upon 365 nm photolysis, a series of new lines appeared in the infrared spectrum, with the strongest appearing at 807.8 and 796.7 wn. To determine the grouping of lines to distinct chemical species, secondary photolysis was performed using a low-pressure Hg lamp in combination with various filters. Based on the secondary photolysis behavior, it was determined that the majority of the new lines belong to two distinct chemical species, designated as set A (3047.2, 1482.2, 1459.5, 1396.6, 1349.6, 1268.2, 1237.9, 1170.3, 1108.8, 807.8, 754.1, 605.6, 526.9, 472.7 wn) and set B (3112.7, 1487.6, 1382.6, 1257.7, 1229.1, 1034.8, 975.8, 942.4, 796.7, 667.9, 569.7 wn). The most likely reactions to occur between Cl and chem{C_5H_8} under the low temperature conditions in solid $p$-chem{H_2} are the addition of the Cl atom to the four distinct alkene carbon atoms to produce the corresponding chlorine atom addition radicals (chem{ClC_5H_8}). Quantum-chemical calculations were performed at the B3PW91/6-311++G(2d,2p) level of theory for the four possible chem{ClC_5H_8} radicals in order to determine the relative energetics and the predicted harmonic vibrational spectra for each radical. The calculations predict that the addition of Cl to each of the four carbons is exothermic, with relative energies of 0.0, 74.5, 67.4, and 7.9 kJ/mol for the addition to carbons 1 – 4, respectively. When the lines of set A and B are compared to the scaled harmonic vibrational spectra for all four of the possible Cl addition radicals, it is found that the best agreement for set A is with the radical produced by the addition to carbon 4 (1-chloromethyl-2-methylallyl radical) and the best agreement for set B is with the radical produced by addition to carbon 1 (1-chloromethyl-1-methylallyl radical). Therefore, the lines of set A and B are assigned to these radicals, respectively

INFRARED SPECTRA OF THE 1,1-DIMETHYLALLYL AND 1,2-DIMETHYLALLYL RADICALS ISOLATED IN SOLID PARA-HYDROGEN

The reaction of hydrogen atoms (H) with isoprene (\chem{C_5H_8}) in solid $para$-hydrogen ($p$-\chem{H_2}) matrices at 3.2 K has been studied using infrared spectroscopy. The production of H atoms for reaction with \chem{C_5H_8} was essentially a three step process. First, mixtures of \chem{C_5H_8} and \chem{Cl_2} were co-deposited in $p$-\chem{H_2} at 3.2 K for several hours, then the matrix was irradiated with ultraviolet light at 365 nm to produce Cl atoms from the \chem{Cl_2}, and finally the matrix was irradiated with infrared light to induce the reaction of the Cl atoms with $p$-\chem{H_2} to produce HCl and H atoms. Upon infrared irradiation, a series of new lines appeared in the infrared spectrum, with the strongest lines appearing at 776.0 and 766.7 \wn. To determine the grouping of lines to distinct chemical species, secondary photolysis was performed using a 365-nm light-emitting diode and a low-pressure mercury lamp in combination with filters. Based on the secondary photolysis, it was determined that the majority of the new lines belong to two distinct chemical species, designated as set X (3030.6, 1573.2, 1452.0, 1435.6, 1123.2, 1051.4, 982.7, 922.5, 792.5, 776.0, 699.2, 524.7, 469.0 \wn) and set Y (3110.1, 2972.0, 1564.4, 1471.1, 1430.2, 1379.7, 1376.2, 1335.4, 1233.0, 1205.4, 1050.1, 766.7, 570.0 \wn). The most likely reactions to occur under the low temperature conditions in solid $p$-\chem{H_2} are the addition of the H atom to the four alkene carbon atoms to produce the corresponding hydrogen atom addition radicals (\chem{HC_5H_8}). Quantum-chemical calculations were performed at the B3PW91/6-311++G(2d,2p) level for the four possible \chem{HC_5H_8} radicals in order to determine the relative energetics and the predicted vibrational spectra for each radical. The addition of H to each of the four carbons is exothermic, with relative energies of 0.0, 93.3, 77.0, and 8.4 kJ/mol for the addition to carbons 1 – 4, respectively. When the lines in set X and Y are compared to the scaled harmonic and anharmonic vibrational spectra, the best agreement for set X is with the radical produced by the addition to carbon 4 (1,2-dimethylallyl radical) and the best agreement for set Y is with the radical produced by addition to carbon 1 (1,1-dimethylallyl radical)

OBSERVATION OF THE C6H7 RADICAL IN AN ARGON MATRIX USING MATRIX ISOLATION INFRARED SPECTROSCOPY

The cyclohexadienyl radical (\chem{C_6H_7}) was observed in a low temperature argon matrix with matrix isolation infrared spectroscopy. The \chem{C_6H_7} radical was produced from the reaction of H atoms with benzene (\chem{C_6H_6}) in the argon matrices. The H atoms were produced by vacuum ultraviolet (VUV) photolysis of \chem{H_2S}, which was co-deposited with the \chem{C_6H_6} in the argon matrices. The most intense peak of the \chem{C_6H_7} radical was observed at 621.0 \wn, with several other weaker peaks observed at 865.9, 910.9, 961.2, 973.7, 1290.3, 1390.2, 1394.9, 1425.9, 2758.7, and 2781.3 \wn. The experiments were performed with various concentrations of \chem{H_2S} and \chem{C_6H_6} and at deposition temperatures of 10 K, 15 K, and 20 K. The largest yield of the \chem{C_6H_7} radical was for VUV photolysis co-deposition of 1:200 \chem{H_2S}:Ar with 1:200 \chem{C_6H_6}:Ar at 15 K. The identification and assignment of the \chem{C_6H_7} radical peaks was accomplished by comparisons to spectra without VUV photolysis, the \chem{H_2S} and \chem{C_6H_6} monomer spectra both with and without VUV photolysis, filtered (400 – 900 nm) Hg-Xe lamp photolysis, and 35 K annealing spectra. Experiments were also performed in which H atoms were reacted with \chem{C_6D_6} producing the \chem{C_6D_6H} radical, with peaks observed at 460.0, 747.8, 759.3, 830.0, 1245.6, 1246.7, and 2791.9/2797.0 \wn. Quantum chemistry calculations for the \chem{C_6H_7} radical were also performed using density functional theory at the B3LYP/aug-cc-pVTZ level to obtain the theoretical structure and theoretical infrared spectrum to support the assignments. The peaks of the \chem{C_6H_7} radical observed in argon matrices are in good agreement with the values reported in xenon matrices\footnote{V.~I.~Feldman, F.~F.~Sukhov, E.~A.~Logacheva, A.~Y.~Orlov, I.~V.~Tyulpina, and D.~A.~Tyurin, Chem.~Phys.~Lett.~\underline{437}, 207 (2007)} and $para$-hydrogen matrices\footnote{M.~Bahou, Y.~J.~Wu, and Y.~P.~Lee, J.~Chem.~Phys.~\underline{136}, 154304 (2012)}