Internal Rotation Barrier Research Articles

Effect of the Chemical Composition and Geometry of Nanotubes on the Structure and Internal Rotation Barrier of Encapsulated Ammonia‐Boron Trifluoride: A Theoretical Investigation

AbstractThe study of the structure and conformational properties of ammonia–boron trifluoride in the cavity of 24 carbon and inorganic nanotubes using the PBE/3ζ DFT approximation showed that the conformational preference of the encapsulated molecule is determined by geometry and chemical composition of the host nanostructure. The combined effect of these factors leads to the predominance of eclipsed conformation of guest molecule for four endoclusters. In other cases, the main minimum on the potential energy surface corresponds to the staggered or pseudo‐staggered forms of BF3←NH3. The guest molecule carries the electric charge and has a shortened B←N bond. These conclusions are confirmed by the results of the virtual reaction of ammonia and boron trifluoride in the cavity of individual nanotubes.

The barrier to internal rotation in silabutenes: A density functional theory study

The potential energy surfaces (PESs) for internal were systematically investigated by using DFT with the B3LYP function. QTAIM, NBO, NEDA and BLW-ED were employed to study the origin of rotational barriers. The silabutenes with M1 = C–M2–M3 skeleton preferred the ac conformer and the M1 = Si–M2–M3 skeletons have the sp conformer. Silabutenes including SiSi double bond have three stable conformers, two different ac conformer and one gauche conformer. Internal rotation barriers (0.38–2.47 kcal/mol) vary with bond lengths (SiC, SiSi, CC), decomposed into steric, electrostatic, and hyperconjugation terms using NSA, NEDA, and NBO. A linear correlation (R2 = 0.78) links barrier heights to energetic differences between stable and metastable conformers. Understanding these factors aids in predicting and rationalizing rotational barriers in silabutenes, crucial for their chemical behavior and applications.

The effect of microsolvation on the structure, nuclear quadrupole coupling, and internal rotation: The methyl carbamate⋯(H2O)1-3 complexes.

Microsolvation of the carbamate moiety delivers precise information on complexation effects on the N-C=O backbone and is of relevance to the peptide bond functionality. In this context, the mono-, di-, and trihydrated complexes of methyl carbamate have been studied in molecular expansion by high-resolution microwave spectroscopy, using chirped-pulse and Fabry-Perot resonator Fourier transform microwave instruments covering the frequency range from 2 to 18GHz. From the rotational constants of the parent and the 18Ow substituted monoisotopologues, accurate values have been derived for the geometries of the hydrogen bond interactions. The nuclear quadrupole coupling constant χcc of the nitrogen nucleus provides a direct measure of complexation changes and decreases with the degree of hydration, whereas the hindered internal rotation barrier increases slightly with microsolvation. Both tendencies could have a common origin in the π-cooperative inductive effects as the microsolvation series progresses. All transitions are split by the internal rotation of the methyl top and the nuclear quadrupole coupling, and in the largest cluster, they are additionally split by an inversion motion.

Detection of a C4 Criegee Intermediate: Fourier-Transform Microwave Spectroscopy of Methacrolein Oxide.

Pure rotational transitions of methacrolein oxide (MACRO) were observed by Fourier-transform microwave spectroscopy. Among the four low-lying conformers existing within an energy window of 3 kcal/mol, only the lowest-energy conformer, the anti-trans conformer, was detected in a discharged jet of a 1,3-diiode-2-methylprop-1-ene and O2 mixture diluted in Ar. Nineteen pure rotational transitions, in the frequency range from 10 to 25 GHz, most of them showing A/E splitting due to the methyl-top internal rotation, were observed and analyzed by the XIAM program, yielding the internal rotation barrier of 559 cm-1, which very well agrees with a theoretically calculated value, 558 cm-1, at the CCSD(T)/cc-pVTZ level of theory.

Equilibrium structures of selenium compounds: The torsionally flexible molecule of selenophenol.

The equilibrium structure of selenophenol has been investigated using rotational spectroscopy and high-level quantum mechanical calculations, offering electronic and structural insight into the scarcely studied selenium compounds. The jet-cooled broadband microwave spectrum was measured in the 2-8GHz cm-wave region using broadband (chirped-pulse) fast-passage techniques. Additional measurements up to 18GHz used narrow-band impulse excitation. Spectral signatures were obtained for six isotopic species of selenium (80Se, 78Se, 76Se, 82Se, 77Se, and 74Se), together with different monosubstituted 13C species. The (unsplit) rotational transitions associated with the non-inverting μa-dipole selection rules could be partially reproduced with a semirigid rotor model. However, the internal rotation barrier of the selenol group splits the vibrational ground state into two subtorsional levels, doubling the dipole-inverting μb transitions. The simulation of the double-minimum internal rotation gives a very low barrier height (B3PW91: 42cm-1), much smaller than for thiophenol (277cm-1). A monodimensional Hamiltonian then predicts a huge vibrational separation of 72.2GHz, justifying the non-observation of μb transitions in our frequency range. The experimental rotational parameters were compared with different MP2 and density functional theory calculations. The equilibrium structure was determined using several high-level abinitio calculations. A final Born-Oppenheimer (reBO) structure was obtained at the coupled-cluster CCSD(T)_ae/cc-wCVTZ level of theory, including small corrections for the wCVTZ → wCVQZ basis set enlargement calculated at the MP2 level. The mass-dependent method with predicates was used to produce an alternative rm(2) structure. The comparison between the two methods confirms the high accuracy of the reBO structure and offers information on other chalcogen-containing molecules.

Nonaxisymmetrical 9,10-diarylanthracenes: Separable swivel conformers and promising optoelectronic properties

The chemical and physical functions of organic molecules are closely related to their conformations in both solutions and solid states, but the successful separation of organic conformers is usually very difficult because of the rapid internal rotation of single bonds. In the current work, we first demonstrate two stable and separable conformers with swivel cross-conjugated configuration based on 9,10-diphenylanthracene derivatives, and they are confirmed by 1H NMR spectra and single crystal analysis. The introduction of two bulky groups at the ortho positions of the two phenyls can enhance the internal rotation barrier of phenyl–anthryl single bond to fully stabilize the conformers in respective conformation even in reflux toluene. We find that they show similar solution but very different solid-state optical properties such as different emission wavelength and fluorescence efficiency. Moreover, only the syn conformer crystal can exhibit mechanochromic luminescence and mechanoluminescence.

Microwave measurements, calculations, and analysis for the gas phase ammonia-formic acid dimer

The gas-phase doubly hydrogen-bonded formic acid-ammonia complex was obtained by heating a sample of ammonium formate in neon. High-level DFT and MP2 calculations with various basis sets were performed and barriers for proton tunneling and internal rotation were determined. The microwave spectrum was measured in the 7–16 GHz frequency range using a Flygare-Balle type pulsed beam Fourier transform microwave (FTMW) spectrometer. Double resonance transitions were observed near 20 GHz. Rotational transitions were measured and fitted for two vibrational states to obtain the rotational constants and quadrupole coupling constants. The rotational constants were determined to have the following values: A = 12017.0(2), B = 4337.331(2), and C = 3227.279(2) for the 0+ state and A = 12017.0, B = 4302.02(1), and C = 3238.161(1) for the 0− state and have been fit to 16 transitions and 9 transitions respectively. Each state was fit to a rigid model showing quadrupole splitting and compared with computations.

Revealing Internal Rotation and 14N Nuclear Quadrupole Coupling in the Atmospheric Pollutant 4-Methyl-2-nitrophenol: Interplay of Microwave Spectroscopy and Quantum Chemical Calculations.

The structure and interactions of oxygenated aromatic molecules are of atmospheric interest due to their toxicity and as precursors of aerosols. Here, we present the analysis of 4-methyl-2-nitrophenol (4MNP) using chirped pulse and Fabry-Pérot Fourier transform microwave spectroscopy in combination with quantum chemical calculations. The rotational, centrifugal distortion, and 14N nuclear quadrupole coupling constants of the lowest-energy conformer of 4MNP were determined as well as the barrier to methyl internal rotation. The latter has a value of 106.4456(8) cm-1, significantly larger than those from related molecules with only one hydroxyl or nitro substituent in the same para or meta positions, respectively, as 4MNP. Our results serve as a basis to understand the interactions of 4MNP with atmospheric molecules and the influence of the electronic environment on methyl internal rotation barrier heights.

Spectroscopy of cluster aerosol models: IR and UV spectra of hydrated glyoxylate with and without sea salt.

Glyoxylic acid is formed in the troposphere by oxidation of organic molecules. In sea salt aerosols, it is expected to be present as glyoxylate, integrated into the salt environment and strongly interacting with water molecules. In water, glyoxylate is in equilibrium with its gem-diol form. To understand the influence of water and salt on the photophysics and photochemistry of glyoxylate, we generate small model clusters containing glyoxylate by electrospray ionization and study them by Fourier-Transform Ion Cyclotron Resonance (FT-ICR) mass spectrometry. We used infrared multiple photon dissociation spectroscopy and UV/vis photodissociation spectroscopy for structural characterization as well as quantum chemical calculations to model the spectra and dissociation patterns. Resonant absorption of infrared radiation leads to water evaporation, which indicates that water and glyoxylate are separate molecular entities in a significant fraction of the clusters, in line with the observed absorption of UV light in the actinic region. Hydration of glyoxylate leads to a change of the dihedral angle in the CHOCOO-·H2O complex, causing a slight redshift of the S1 ← S0 transition. However, the barriers for internal rotation are below 5 kJ mol-1, which explains the broad S1 ← S0 absorption extending from about 320 to 380 nm. Most importantly, hydration hinders dissociation in the S1 state, thus enhancing the quantum yield of fluorescence combined with water evaporation. No C-C bond photolysis is observed, but due to the limited signal-to-noise ratio, it cannot be ruled out. The quantum yield, however, will be relatively low. Fluorescence dominates the photophysics of glyoxylate embedded in the dry salt cluster, but the quantum yield shifts towards internal conversion upon addition of one or two water molecules.

Fluorination effects probed in 4-fluoroacetophenone and its monohydrate.

Rotational spectra of the 4-fluoroacetophenone monomer and its monohydrate were investigated by Fourier transform microwave spectroscopy complemented with quantum chemical calculations. One conformer of 4-fluoroacetophenone and two isomers of 4-fluoroacetophenone-H2O have been observed in the pulsed jets. The observation of all mono-substituted 13C isotopologues in natural abundance allows an accurate structural determination of the 4-fluoroacetophenone monomer. Both detected isomers of 4-fluoroacetophenone-H2O are stabilized by a dominant O-H⋯O and a secondary C-H⋯O hydrogen bond. The fluorination effects on the geometries, intermolecular non-covalent interactions and V3 barrier of the methyl internal rotation were analysed. The relative population ratio of the two observed isomers for 4-fluoroacetophenone-H2O was also estimated to be NI/NII ≈ 7/1.

Microwave spectra of dinitrotoluene isomers: a new step towards the detection of explosive vapors.

The spectroscopic characterization of explosive taggants used for TNT detection is a research topic of growing interest. We present a gas-phase rotational spectroscopic study of weakly volatile dinitrotoluene (DNT) isomers. The pure rotational spectra of 2,4-DNT and 2,6-DNT were recorded in the microwave range (2-20 GHz) using a Fabry-Perot Fourier-transform microwave (FP-FTMW) spectrometer coupled to a pulsed supersonic jet. Rotational transitions are split by hyperfine quadrupole coupling at the two 14N nuclei leading to up to 9 hyperfine components. The spectral analysis was supported by quantum chemical calculations carried out at the B98/cc-pVTZ and MP2/cc-pVTZ levels of theory. Based on 2D potential energy surfaces at the B98/cc-pVTZ level of theory, the methyl group internal rotation barriers were calculated to be V3 = 515 cm-1 and 698 cm-1 for 2,4- and 2,6-DNT, respectively. Although no splitting due to internal rotation was observed for 2,6-DNT, several splittings were observed for 2,4-DNT. The microwave spectra of both species were fitted using a semi-rigid Hamiltonian accounting for the quadrupole coupling hyperfine structure. Based on the internal axis method (IAM), an additional analysis was performed to retrieve an accurate value of the rotationless A-E tunneling splitting which could be extracted from the rotational dependence of the tunneling splitting. This yielded in the case of 2,4-DNT to an experimental value of 525 cm-1 for the barrier height V3 which agrees well with the DFT value. The coupled internal rotations of -CH3 and -NO2 are investigated in terms of 2-D surfaces, as already done in the case of 2-nitrotoluene [A. Roucou et al., Chem. Phys. Chem., 2020, 21, 2523-2538].

On the origin and nature of internal methyl rotation barriers: an information-theoretic approach study

On the origin and nature of internal methyl rotation barriers: an information-theoretic approach study

Internal dynamics of methyl p-tolyl sulfoxide in the gas phase: Rotational spectroscopy and theoretical studies.

A pure rotational spectrum of methyl p-tolyl sulfoxide (MTSO) was studied using chirped-pulse Fourier transform microwave spectroscopy in the frequency range of 18-26GHz. A single conformer was unambiguously observed in the supersonic jet expansion, which is consistent with the conformational analysis performed using quantum-chemical calculations. Rotational transitions were split into two components of A and E symmetries due to the low-barrier internal rotation of the ring methyl group [V3 = 11.0178(23) cm-1]. The low energy barrier for the methyl top internal rotation implies an electron-withdrawing effect of the group at the opposite side of the phenyl ring, in comparison with other para-substituted toluenes. The effective ground state (r0) geometry was derived using the rotational constants from the parent species and the 34S and eight 13C singly substituted isotopologues. Compared to two other sulfoxides, methyl phenyl sulfoxide and methyl 4-nitrophenyl sulfoxide, the sulfoxide group in MTSO is slightly more twisted with respect to the plane of the phenyl ring, which could be attributed to the moderate electron-donating effect of the p-methyl group. Furthermore, the pyramidal inversion that interconverts the handedness at the sulfur stereogenic center was explored in the electronic ground (S0) and excited (S1) states with nudged elastic band and time-dependent density functional theory methods. It was found that the pyramidal inversion in S1 is easier than in S0, showing that optical excitation to S1 will facilitate an effectively barrier-free inversion.

Theoretical spectroscopic study of acetyl (CH 3CO), vinoxy (CH 2CHO), and 1-methylvinoxy (CH 3COCH 2) radicals. Barrierless formation processes of acetone in the gas phase.

Background: Acetone is present in the earth´s atmosphere and extra-terrestrially. The knowledge of its chemical history in these environments represents a challenge with important implications for global tropospheric chemistry and astrochemistry. The results of a search for efficient barrierless pathways producing acetone from radicals in the gas phase are described in this paper. The spectroscopic properties of radicals needed for their experimental detection are provided. Methods: The reactants were acetone fragments of low stability and small species containing C, O and H atoms. Two exergonic bimolecular addition reactions involving the radicals CH 3, CH 3CO, and CH 3COCH 2, were found to be competitive according to the kinetic rates calculated at different temperatures. An extensive spectroscopic study of the radicals CH 3COCH 2 and CH 3CO, as well as the CH 2CHO isomer, was performed. Rovibrational parameters, anharmonic vibrational transitions, and excitations to the low-lying excited states are provided. For this purpose, RCCSD(T)-F12 and MRCI/CASSCF calculations were performed. In addition, since all the species presented non-rigid properties, a variational procedure of reduced dimensionality was employed to explore the far infrared region. Results: The internal rotation barriers were determined to be V 3=143.7 cm -1 (CH 3CO), V 2=3838.7 cm -1 (CH 2CHO) and V 3=161.4 cm -1 and V 2=2727.5 cm -1 (CH 3COCH 2).The splitting of the ground vibrational state due to the torsional barrier have been computed to be 2.997 cm -1, 0.0 cm -1, and 0.320 cm -1, for CH 3CO, CH 2CHO, and CH 3COCH 2, respectively. Conclusions: Two addition reactions, H+CH 3COCH 2 and CH 3+CH 3CO, could be considered barrierless formation processes of acetone after considering all the possible formation routes, starting from 58 selected reactants, which are fragments of the molecule. The spectroscopic study of the radicals involved in the formation processes present non-rigidity. The interconversion of their equilibrium geometries has important spectroscopic effects on CH 3CO and CH 3COCH 2, but is negligible for CH 2CHO.

Water binding to the atmospheric oxidation product methyl vinyl ketone

Methyl vinyl ketone is one of the major oxidation products of isoprene, and therefore, an important precursor of secondary organic aerosol. Understanding its interactions with water is relevant to gain insight into aerosol formation and improve the predictive power of atmospheric chemistry models. The molecular complex formed between methyl vinyl ketone and water has been generated in a supersonic jet and characterized using high-resolution microwave spectroscopy in combination with quantum chemistry calculations. In this study, we show that methyl vinyl ketone interacts with water forming four 1:1 isomers connected by O − H···O and C − H···O hydrogen bond interactions. Water has been found to preferentially bind to the antiperiplanar conformation of methyl vinyl ketone. Evidence of a large amplitude motion arising from the methyl internal rotation has been found in the rotational spectra of the dimer. The threefold methyl internal rotation barrier heights have been further determined and discussed for all the species.

Theoretical spectroscopic study of acetyl (CH3CO), vinoxy (CH2CHO), and 1-methylvinoxy (CH3COCH2) radicals. Barrierless formation processes of acetone in the gas phase

Background: Acetone is present in the earth´s atmosphere and extra-terrestrially. The knowledge of its chemical history in these environments represents a challenge with important implications for global tropospheric chemistry and astrochemistry. The results of a search for efficient barrierless pathways producing acetone from radicals in the gas phase are described in this paper. The spectroscopic properties of radicals needed for their experimental detection are provided. Methods: The reactants were acetone fragments of low stability and small species containing C, O and H atoms. Two exergonic bimolecular addition reactions involving the radicals CH3, CH3CO, and CH3COCH2, were found to be competitive according to the kinetic rates calculated at different temperatures. An extensive spectroscopic study of the radicals CH3COCH2 and CH3CO, as well as the CH2CHO isomer, was performed. Rovibrational parameters, anharmonic vibrational transitions, and excitations to the low-lying excited states are provided. For this purpose, RCCSD(T)-F12 and MRCI/CASSCF calculations were performed. In addition, since all the species presented non-rigid properties, a variational procedure of reduced dimensionality was employed to explore the far infrared region. Results: The internal rotation barriers were determined to be V3=143.7 cm-1 (CH3CO), V2=3838.7 cm-1 (CH2CHO) and V3=161.4 cm-1 and V2=2727.5 cm-1 (CH3COCH2).The splitting of the ground vibrational state due to the torsional barrier have been computed to be 2.997 cm-1, 0.0 cm-1, and 0.320 cm-1, for CH3CO, CH2CHO, and CH3COCH2, respectively. Conclusions: Two addition reactions, H+CH3COCH2 and CH3+CH3CO, could be considered barrierless formation processes of acetone after considering all the possible formation routes, starting from 58 selected reactants, which are fragments of the molecule. The spectroscopic study of the radicals involved in the formation processes present non-rigidity. The interconversion of their equilibrium geometries has important spectroscopic effects on CH3CO and CH3COCH2, but is negligible for CH2CHO.

Structure and dynamics of methacrylamide, a computational and free-jet rotational spectroscopic study

The conformational space of methacrylamide was explored by quantum mechanical modeling and surveyed in the 59.6–104.0 GHz frequency range using a millimeter-wave Stark-modulated free-jet absorption spectrometer. According to the relative orientation of the two unsaturated bonds, two conformers were observed, namely s-trans (A=5234.360(1), B=3364.9717(8) and C=2173.099(1) MHz) and s-cis (A=5207.292(1), B=3470.930(1) and C=2113.496(1) MHz). The s-trans conformation is the global minimum, with relative energy 4(2) kJ mol−1 and calculated isomerization barrier 15 kJ mol−1. Except for the methyl hydrogen atoms, s-cis-methacrylamide is planar and its methyl internal rotation barrier is 10.2(1) kJ mol−1. In s-trans-methacrylamide the allyl and amino frames form a dihedral angle of about 30° and the methyl internal rotation barrier is 7.4 kJ mol−1. This different behaviour is explained in terms of attractive and repulsive intramolecular interactions between groups: CH2/CO and CH3/NH2 for s-cis, CH2/NH2 and CH3/CO for s-trans. The tunneling splitting related to the double-well potential describing the interconversion between the two equivalent s-trans forms is 837.97(2) MHz and was reproduced by a one-dimensional flexible model using a 3.6 kJ mol−1 interconversion barrier.

Electron interactions with hydrogen peroxide (H2O2)

Hydrogen peroxide (H2O2) is an important compound in several chemical processes that take place in outer space. As the molecule presents a considerable low θHOOH internal rotational barrier regarding the interconversion to the trans structure, such conformation may be accessed when at the stratosphere. Hence, in this work, a systematic computational investigation on the electron interactions with H2O2 was performed through exploring the differential cross sections (DCS), integral cross sections (ICS), and momentum transfer cross sections (MTCS) for elastic scattering as well as the total cross sections (TCS) for different conformations of the target (θHOOH=119.8° - experimental structure and θHOOH=180° - trans) at a wide energy range (1–500 eV). To describe the electron-target interaction and solve the scattering equations, a molecular complex optical potential approach combined with Padé approximants was employed. At high energies, excellent agreement between present ICS (and also TCS) results and those determined in a previous work (through the use of a different methodology) was observed. In addition, ICS and MTCS were investigated extensively for H2O2. Interestingly, a resonance-like feature was observed in the ICS and MTCS of both conformations studied, which was assigned to the OO bond. The resonance probed has σ∗ character and is related to the B continuum (in the case of the experimental conformation) and Bu scattering symmetry (in trans conformation). More important, a shift of 1 eV was determined regarding the peak probed for trans conformation in comparison to that observed for the conformation with non-zero dipole moment, which may be used as an auxiliary for helping the identification of the presence of different H2O2 conformation in future experimental investigations. We encourage other investigations to be performed to verify (and to contribute to) the findings achieved in this work.

Large Amplitude Torsions in Nitrotoluene Isomers Studied by Rotational Spectroscopy and Quantum Chemistry Calculations.

Rotational spectra of ortho-nitrotoluene (2-NT) and para-nitrotoluene (4-NT) have been recorded at low and room temperatures using a supersonic jet Fourier Transform microwave (MW) spectrometer and a millimeter-wave frequency multiplier chain, respectively. Supported by quantum chemistry calculations, the spectral analysis of pure rotation lines in the vibrational ground state has allowed to characterise the rotational energy, the hyperfine structure due to the 14 N nucleus and the internal rotation splittings arising from the methyl group. For 2-NT, an anisotropic internal rotation of coupled -CH3 and -NO2 torsional motions was identified by quantum chemistry calculations and discussed from the results of the MW analysis. The study of the internal rotation splittings in the spectra of three NT isomers allowed to characterise the internal rotation potentials of the methyl group and to compare them with other mono-substituted toluene derivatives in order to study the isomeric influence on the internal rotation barrier.

Theoretical study of internal rotational barriers of electrons donating and electrons withdrawing groups in aromatic compounds

The presence of internal rotation in sigma bonds is essential for conformational analysis of organic molecules and its understanding is of great relevance in chemistry, as well as in several other areas. However, for aromatic compounds that have substituent groups, withdrawers or donors of electron, there are no data in the literature to explain their rotational barriers. In this context, the work studied the internal rotational barriers of electron donating and withdrawing groups in aromatic compounds using the MP3, MP4, and CCSD(T) methods and the influence of substituents' nature on barrier heights was investigated through calculations based on the theory of Natural Bond Orbitals (NBO) and Quantum Theory of Atoms in Molecules (QTAIM). The results obtained showed that the CCSD(T) method is the one that best describes the internal rotational barriers, followed by MP4 and MP3 and the electron donating groups decrease the barrier, whereas electron withdrawing groups increase. Through the NBO analysis it was possible to observe that for withdrawing groups the interaction of the molecular orbitals is more accentuated promoting the increase of the rotational barrier of these compounds. Through the QTAIM analysis it was possible to show that, for electron donating groups, the internal rotation is influenced by the loss of electronic density when the substituents is perpendicular to the ring plane, however, for withdrawing groups the density is little influenced, regardless of the two conformations (minimum and maximum energy). Two molecules showed free rotation, trichloromethylbenzene and methylbenzene, and the theoretical calculations NBO and QTAIM showed that for these species there is no difference in the properties studied when there is rotation of the dihedral angle.