Internal Rotation Splittings Research Articles

The Effect of Hyperconjugation and Hydrogen Bonding on the Conformers of Methylated Monosaccharides.

The conformational landscapes of four 1-O-methylated monosaccharides-methyl a-glucose, methyl b-glucose, methyl a-galactose, and methyl b-galactose-were characterized using jet-cooled broadband rotational spectroscopy, supported by density functional theory calculations. A newly designed, simple pulsed nozzle assembly was used to introduced the sugar samples into a jet expansion without thermal degradation, eliminating the need for a complex and expensive laser ablation system. Ten conformers were experimentally identified by assigning their rotational spectra, and the intricate methyl internal rotation splittings were analysed. Notably, methylation alters the directionality of intramolecular hydrogen bonding of a-galactose highlighting its impact on structural preference. Natural bond orbital, intrinsic bond strength, and non-covalent interaction analyses were conducted to explore the interplay between hydrogen bonding and hyperconjugation. A set of σ to σ* neutral hyperconjugative interactions were found to override a strong hydrogen bond, driving a preference for the gauche conformers.

Approaching the free rotor limit: extremely low methyl torsional barrier observed in the microwave spectrum of 2,4-dimethylfluorobenzene.

The microwave spectrum of 2,4-dimethylfluorobenzene was recorded using a molecular jet Fourier transform microwave spectrometer in the frequency range from 2.0 to 26.5 GHz. The spectral assignment and modeling were challenging due to the large tunnelling splittings resulting from the very low barrier to internal rotation of the p-methyl group that approaches the free rotor limit. Internal rotation splittings arising from two inequivalent o- and p-methyl groups were observed, analysed and modelled using the modified version of the XIAM code and the BELGI-Cs-2Tops code, giving a root-mean-square deviation of 549.1 kHz and 4.5 kHz, respectively, for a data set of 885 rotational lines. The torsional barriers of the o- and p-methyl groups were determined to be 227.039(51) cm-1 and 3.23(40) cm-1, respectively. The V3 barrier observed for the p-methyl group is lower than in any other para-methyl substituted toluene derivatives with coupled internal rotations, becoming the lowest value ever observed to date. The barrier to internal rotation of the o-methyl group next to a fluorine atom is consistently around 220 cm-1, as confirmed by comparing it to barriers observed in other toluene derivatives. The experimental rotational constants were compared to those obtained by quantum chemical calculations.

Rotational spectroscopy of methyl tert-butyl ether with a new Ka band chirped-pulse Fourier transform microwave spectrometer.

Chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy is a powerful tool for performing broadband gas-phase rotational spectroscopy, and its applications include discovery of new molecules, complex mixture analysis, and exploration of fundamental molecular physics. Here we report the development of a new Ka band (26.5-40 GHz) CP-FTMW spectrometer that is equipped with a pulsed supersonic expansion source and a heated reservoir for low-volatility samples. The spectrometer is built around a 150 W traveling wave tube amplifier and has an instantaneous bandwidth that covers the entire Ka band spectral range. To test the performance of the spectrometer, the rotational spectrum of methyl tert-butyl ether (MTBE), a former gasoline additive and environmental pollutant, has been measured for the first time in this spectral range. Over 1000 spectroscopic transitions have been measured and assigned to the vibrational ground state and a newly-identified torsionally excited state; all transitions were fit using the XIAM program to a root-mean-square deviation of 22 kHz. The spectrum displays internal rotation splitting, nominally forbidden transitions, and an intriguing axis-switching effect between the ground and torsionally excited state that is a consequence of MTBE's extreme near-prolate nature. Finally, the sensitivity of the spectrometer enabled detection of all singly-substituted 13C and 18O isotopologues in natural abundance. This set of isotopic spectra allowed for a partial r0 structure involving the heavy atoms to be derived, resolving a structural discrepancy in the literature between previous microwave and electron diffraction measurements.

Rotational Rest Frequencies and First Astronomical Search of Protonated Methylamine

We report first laboratory rest frequencies for rotational transitions of protonated methylamine, CH3NH3+, measured in a cryogenic 22-pole ion trap machine and employing an action spectroscopy scheme. For this prolate symmetric top molecule thirteen transitions between 80 and 240 GHz were detected in the ground vibrational state, covering JK = 2K − 1K up to JK = 6K − 5K with K = 0, 1, 2. Some transitions exhibit noticeable structure that is attributed to internal rotation splitting. As the CN radical and several of its hydrogenated and protonated forms up to methylamine, CH3NH2, are well known entities in the laboratory and in space, protonated methylamine, CH3NH3+, is a promising candidate for future radio astronomical detection.

A prochiral precursor in space? Accurate laboratory characterization of acetylacetylene in the cm-wave region

The most prominent pure rotational transitions of the prochiral, possibly prebiotic molecule acetylacetylene are recorded in the frequency range of 6.5–26.5 GHz for a reliable spectral characterization prerequisite to a possible extraterrestrial observation. Therefore, three different approaches for global fitting of the vibrational ground state are applied to analyze the experimentally determined transition frequencies using the programs SPFIT, XIAM and ERHAM corresponding to different theoretical treatments solving the internal rotation problem.Furthermore, the resulting internal rotation parameter V3 is compared to those other substituted acetyl species, and derivatives of butynes and possible explanations for the observed trends are given. For the substituted acetyl species, the spectra of acetylfluoride, -chloride, -bromide and -iodide are reanalyzed.The obtained rotational constants have been interpreted in terms of characterizing the geometric structure. A correlation between the tunneling parameter ε1 and V3 of several molecular species displaying spectral internal rotation splitting patterns has been found and quantified. This relationship can simplify the analysis of vibrational ground state rotational spectra in finding a suitable starting point for the fitting procedure using the program ERHAM, which is the predestined choice to analyze and predict signal positions up to very high quantum numbers. Being able to predict those signal positions is crucial for a possible comprehensive astronomical identification and subsequent application as an astrophysical probe for the conditions in interstellar molecular clouds.

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.

Detection of a Criegee Intermediate with an Unsaturated Hydrocarbon Substituent: Fourier-Transform Microwave Spectroscopy of Methyl Vinyl Ketone Oxide.

Pure rotational transitions of methyl vinyl ketone oxide, or the so called methyl-vinyl Criegee intermediate, were observed by Fourier-transform microwave spectroscopy. Among the four possible conformers of this species, predicted by theory within an energy window of 3 kcal/mol, only the lowest-energy conformer, the syn-trans form, was detected in a discharged jet of a 1,3-diiode-but-2-ene (either in Z- or E-conformation) and O2 mixture diluted in Ar. Thirty pure rotational transitions with internal rotation splitting of the methyl top were observed. The observed frequencies were analyzed by the XIAM program, yielding an internal rotation barrier of 702.8(28) cm-1, which reasonably agrees with a theoretical value of 680 cm-1 determined with CCSD(T)/cc-pVTZ.

Barriers to internal rotation and tunnelling splittings of the torsional states in the HO(CH2)OH, DO(CH2)OH and DO(CH2)OD molecules

Torsional states caused by vibrations of hydroxyl groups in the methanediol molecule and its two deuterated analogues – DO(CH2)OH and DO(CH2)OD were analysed at MP2/cc-pVTZ and CCSD(T)/cc-pVQZ levels of theory. In the first case, 2D PES and 2D surfaces of kinematic coefficients were calculated with geometry optimisation for all other geometric parameters, and in the second case, only the energy of optimised configurations at the MP2/cc-pVTZ level of theory was determined. Then 2D PES was recounted to the complete basis set (CBS) limit by extrapolating the results of calculations at the MP2/cc-pVTZ and MP2/cc-pVQZ levels of theory The calculated values were then averaged over four equivalent points on the coordinate plane. Hamiltonian matrices were constructed using DVR and Fourier methods. After their subsequent diagonalization, the energies of the stationary torsional states were computed. Their classification by C2V(M) and CS(M) molecular symmetry groups has been performed. The splitting values due to the tunnelling of the thirty most deeply located torsional states in the three studied molecules were also determined. The torsional states, internal rotation barriers, and tunnelling frequencies in the molecules of methanediol and hydrogen trioxide were compared.

Broadband rotational spectroscopy of trans 3-pentenenitrile and 4-pentenenitrile.

Titan, a moon of Saturn, has a nitrogen- and methane-rich atmosphere that is similar to prebiotic earth, and is replete with organic nitriles. Pentenenitriles have not yet been detected in Titan's atmosphere or in molecular clouds, but are potential precursors to hetero-aromatic compounds such as pyridine. We performed broadband microwave studies in the 8-18 GHz range on the trans isomer of 3-pentenenitrile (3-PN) and 4-pentenenitrile (4-PN) under jet-cooled conditions. Strong-field coherence breaking (SFCB) was used to selectively modulate the intensities of microwave transitions in a conformer-specific manner for 3-PN, aiding analysis. Two conformers of 3-PN and five conformers of 4-PN were identified and the rotational transitions were assigned. Evidence for methyl internal rotation splitting was observed for both the conformers of 3-PN, and the barrier heights of both conformers was determined experimentally. Comparison is made of the conformational preferences, stability and isomerization barriers through the acquired rotational spectra and potential energy surface (PES) calculations of the structural isomers 3-PN and 4-PN.

High resolution rovibrational analysis of dimethyl sulfide spectrum in the 10 µm atmospheric window combining supersonic jet-quantum cascade laser and FTIR spectroscopies

The rovibrational spectrum of the v14 CH3-bending mode of dimethyl sulfide CH3SCH3 was recorded in the 945–1005 cm−1 spectral region combining data from a tunable infrared quantum cascade laser spectrometer coupled to a pulsed slit jet and from a high resolution Fourier transform spectrometer coupled to a room temperature gas absorption cell. The characteristic PQR band contour of a vibrational mode with a b1 symmetry species in the C2v group was observed with both infrared methods but the rovibrational cooling achieved in the pulsed supersonic expansion (20 K in rotation) proved to be essential for starting the rovibrational analysis of such asymmetric top molecule. Internal rotation splittings were recently observed and analyzed in the ground and first torsional excited states. However, they are expected to be very small and could not be resolved in the v14 state using our infrared spectroscopic probes. Therefore a classic reduced effective Hamiltonian of an asymmetric rigid rotor was used. Finally, 1119 transitions from both jet-cooled and room temperature experiments were assigned and included in a global fit with J ≤ 29. A total of 9 rovibrational parameters including pure rotational constants as well as quartic centrifugal distortion constants were determined with high accuracy. These parameters can be used to produce a reliable line-list for an atmospheric detection of DMS.

Structure Determination, Conformational Flexibility, Internal Dynamics, and Chiral Analysis of Pulegone and Its Complex with Water.

In the current work we present a detailed analysis of the chiral molecule pulegone, which is a constituent of essential oils, using broadband rotational spectroscopy. Two conformers are observed under the cold conditions of a molecular jet. We report an accurate experimentally determined structure for the lowest energy conformer. For both conformers, a characteristic splitting pattern is observed in the spectrum, resulting from the internal rotation of the two non-equivalent methyl groups situated in the isopropylidene side chain. The determined energy barriers are 1.961911(46) kJ mol-1 and 6.3617(12) kJ mol-1 for one conformer, and 1.96094(74) kJ mol-1 and 6.705(44) kJ mol-1 for the other one. Moreover, a cluster of the lowest energy conformer with one water molecule is reported. The water molecule locks one of the methyl groups by means of a hydrogen bond and some secondary interactions, so that we only observe internal rotation splittings from the other methyl group with an internal rotation barrier of 2.01013(38) kJ mol-1 . Additionally, the chirality-sensitive microwave three-wave mixing technique is applied for the differentiation between the enantiomers, which can become of further use for the analysis of essential oils.

Cross-contamination of the fitting parameters in multidimensional tunneling treatments

In this paper we examine the two-dimensional tunneling formalism used previously to fit the hydrogen-transfer and internal-rotation splittings in the microwave spectrum of 2-methylmalonaldehyde in an effort to determine the origin of various counterintuitive results concerning the isotopic dependence of the internal-rotation splittings in that molecule. We find that the cause of the problem lies in a “parameter contamination” phenomenon, where some of the numerical magnitude of splitting parameters from modes with large tunneling splittings “leaks into” the parameters of modes with smaller tunneling splittings. We show that such parameter contamination, which greatly complicates the determination of barrier heights from the least-squares-fitted splitting parameters, will be a general problem in spectral fits using the multi-dimensional tunneling formalism, since it arises from subtle mathematical features of the non-orthogonal framework functions used to set up the tunneling Hamiltonian. Transforming to a physically less intuitive orthonormal set of basis functions allows us to give an approximate numerical estimate of the contamination of tunneling parameters for 2-methylmalonaldehyde by combining a dominant tunneling path hypothesis with results recently given for the hydrogen-transfer-internal-rotation potential function for this molecule.

Internal rotation in halogenated toluenes: Rotational spectrum of 2,5-difluorotoluene

The microwave spectrum of 2,5-difluorotoluene was investigated in a pulsed Molecular Beam Fourier Transform Microwave (MB-FTMW) spectrometer in the frequency region 4.5–24GHz. Rotational lines show large internal rotation splittings indicating that the potential barrier hindering internal rotation of the methyl group is comparatively low. The torsional splittings could be unambiguously identified and the potential barrier hindering internal rotation of the methyl top was determined. Rotational constants for the parent species, together with quartic centrifugal distortion terms were determined for a total of eight isotopologues, comprising the parent and all monosubstituted 13C species. Supporting ab initio (MP2) and density functional (B3LYP) calculations are provided. The experimental potential barrier V3=2580(12)J/mol compares satisfactorily with the ab initio prediction of 2568Jmol−1.

Structure and internal rotation dynamics of the acetone-neon complex studied by microwave spectroscopy

The microwave spectra of the van der Waals complexes acetone-20Ne and acetone-22Ne were measured using a cavity-based supersonic jet Fourier-transform microwave spectrometer in the region from 5 to 18GHz. For these two isotopologues, both c- and weaker a-type transitions were observed. The transitions are split into multiplets due to the internal rotation of the two methyl groups in acetone. Initial electronic structure calculations were performed at the MP2/6-311++g (2d,p) level of theory and the internal rotation barrier height of the methyl groups was calculated to be ∼2.8kJ/mol. The ab initio rotational constants were the basis for the spectroscopic searches, but the multiplet structures and floppiness of the complex made the quantum number assignment very difficult. The assignment was finally achieved with the aid of constructing closed frequency loops and predicting internal rotation splittings using the XIAM internal rotation program. The acetone methyl group tunneling barrier height was determined experimentally to be 3.10(6)kJmol−1 [259(5)cm−1] in the acetone-Ne complex, which is lower than in the acetone monomer but comparable to the acetone-Ar complex (Kang et al., 2002). Experimental data and high-level CCSD(T)/aug-cc-pVTZ calculations suggest that the Ne atom lies directly above the plane formed by the carbonyl group and the two carbon-carbon bonds, which is different than the slightly offset position found previously in the acetone-Ar complex. Additionally, ab initio calculations and Quantum Theory of Atoms in Molecules analyses were used to analyze the methyl internal rotation motions in acetone and acetone-Ne.

Communication through the phenyl ring: internal rotation and nuclear quadrupole splitting in p-halotoluenes

The rotational spectra of three p-halotoluenes (chloro-, bromo- and iodo-) are reported in the frequency range 2–8.5 GHz, obtained with broadband Fourier-transform microwave spectroscopy. The recorded spectra are highly complicated due to low-barrier V 6 internal rotation of the methyl group as well as strong nuclear quadrupole coupling of the halogen atoms. However, these additional effects allow us, in a comparative manner, to study potential crosstalk of the two substituents via the phenyl ring. The rotational constants and other molecular parameters are reported and compared with quantum chemical calculations. The V 6 internal rotation barrier of the methyl group was found to be 145 GHz for both p-chlorotoluene species. We found that the magnitudes of the quadrupole coupling constants are increased in the halotoluenes compared to the halobenzenes. This increase is due to the +I inductive effect of the methyl group that injects additional electron density into the phenyl π-cloud, thus giving more electron density for the halogen atom to extract. This additional extraction makes the halogen–carbon bond more ionic than in the halobenzenes.

High-resolution spectrum of the ν9 band and reinvestigation of the ν8 band of cis-CH3ONO

The infrared spectrum of methyl nitrite CH3ONO has been recorded at a spectral resolution of 0.003cm−1 using a Fourier-transform spectrometer Bruker IFS125HR. The ν8 band of the cis isomer has been reinvestigated in the 780–880cm−1 spectral range to complete the study made by Goss et al. (2004) [3] and to fit the internal rotor splittings. The BELGI-IR program, which enables us to treat an isolated infrared band for asymmetric molecules containing one internal methyl rotor has been used for the analysis and predictions of spectra. Finally 1036 lines (913 A-type and 123 E-type lines for J≤50 and Ka≤28) have been assigned for the cis isomer and fitted with a standard deviation of 0.00047cm−1.Furthermore, for the first time, the ν9 band of cis-CH3ONO was investigated in the 540–660cm−1 spectral range and rather large internal rotation splittings were also observed at higher J values. For the ν9 band, the effective approach performed with the BELGI-IR program allowed us to analyze and reproduce 682 lines up to J=50 and Ka=18 with a standard deviation of 0.00051cm−1. The multiple vibration–rotation–torsion interactions, which are likely to occur between the excited v9=1 and v8=1 states and the torsional manifolds are discussed.

Conformational behaviour, hydrogen bond competition and intramolecular dynamics in vanillin derivatives: acetovanillone and 6-hydroxy-3-methoxyacetophenone

The conformational landscape of the structural isomers acetovanillone (apocynin, AV) and 6-hydroxy-3-methoxyacetophenone (HMAP) has been investigated in a supersonic jet using Fourier transform microwave spectroscopy. Two conformers have been detected in the jet-cooled expansion for each molecule (s-cis and s-trans in AV; s-trans and a-trans for HMAP), differing in the relative orientation of the acetyl and methoxy groups. Both molecules are stabilized by O-H···O or O-H···O=C hydroxyl intramolecular hydrogen bonds, either constraining the local conformations of the methoxy group in AV, or that of the acetyl group in HMAP. Internal rotation splittings have been observed in both conformers of each molecule, originated by the acetyl group, that yield information on the influence of the intramolecular hydrogen bonds on the methyl torsion. The similar internal rotation barriers in both molecules (6.6 and 7.4 kJ mol(-1) in AV; 7.3 and 7.0 kJ mol(-1) in HMAP) suggest that the acetyl torsion is only slightly affected by intramolecular hydrogen bonding. The absence of torsional tunnellings due to the methoxy group indicates torsional barriers above 10.2 and 8.9 kJ mol(-1) for AV conformers, 10.1 and 10.4 kJ mol(-1) for HMAP. Conformational ratios and relative free energies have been estimated from relative intensity measurements of the spectral lines. Ab initio (MP2) and density functional calculations using the recent M05-2X empirical functional have been used to aid the experimental work in describing the structures, internal rotation barriers and isomerization potentials.

Fourier transform microwave spectrum of the CO–dimethyl sulfide complex

The rotational spectrum of the CO–dimethyl sulfide (DMS) complex was measured in the frequency region from 4.8 up to 25 GHz by Fourier transform microwave spectroscopy. For the normal species 27 a-type and 57 c-type transitions were observed, while 16 and 8 c-type transitions were assigned for the species with 34S and 13C in the DMS moiety, respectively, in natural abundance. In addition, 7 a-type and 48 c-type transitions were assigned for the complex with the 13CO enriched species as a component and 9 a-type and 42 c-type transitions for the complex with enriched C 18O. No splitting was observed, which could be ascribed to the tunneling motion of the CO between two possible potential minima around DMS, while many transitions were split by the internal-rotation of the two methyl tops of the DMS unit. In cases where the K-type splitting was close to the methyl internal-rotation splitting, forbidden transitions were observed which apparently followed b-type selection rules. All of the observed transition frequencies for the normal species were analyzed simultaneously using a two-top internal-rotation and rotation Hamiltonian. The potential barrier height V 3 to internal rotation of the methyl groups of the DMS was determined to be 745.5 (30) cm −1. The transition frequencies observed for all the isotopomers were analyzed using an asymmetric-rotor rotational Hamiltonian, to determine rotational and centrifugal distortion constants. The r s coordinates calculated from the observed rotational constants led to the conclusion that the CO moiety was located in a plane perpendicular to the skeletal plane of the DMS and bisecting its CSC angle. This structure of the CO–DMS is very much different from that of the CO–DME, in which the CO is located in the DME skeletal plane. The distance between the centers of gravity of the two moieties, R cm, was calculated to be 3.789 Å for the CO–DMS, which is longer by only 0.11 Å than that in the CO–DME complex: 3.68 Å, in spite of the fact that the van der Waals radius of the S atom is much larger than that of the O atom. The small difference in R cm is, in part, ascribed to the location of the CO relative to the DMS/DME. The more important reason is that the intermolecular bonding of the CO–DMS is stronger than that of CO–DME; by assuming a Lennard–Jones-type potential, the force constant of the van der Waals stretching mode and the dissociation energy were estimated to be 2.7 Nm −1 and 3.3 kJ mol −1, respectively, which were larger than those of the CO–DME: 1.4 Nm −1 and 1.6 kJ mol −1.

The microwave spectroscopy of trans-ethyl methyl ether in the [formula omitted] excited torsional state

The trans-ethyl methyl ether molecule has two inequivalent methyl group internal rotors. One methyl rotor is bonded to the oxygen atom and the other to the carbon atom. The internal rotations of these methyl rotors correspond to the vibrational modes, ν 29 and ν 28 , respectively. In this study, the microwave absorption spectrum in the ν 29 = 1 excited torsional state was analyzed for the first time. Initial assignment has been carried out by making use of measurements at dry ice temperature. Over 1500 lines up to J = 76 and K = 5 were assigned. The spectrum involving internal rotation splittings was analyzed by using the tunneling matrix formulation developed by Hougen.

Three Intermolecular Bonds Form a Weak but Rigid Complex: O(CH3)2···N2O

The rotational spectrum of the dimethylether (DME)-N(2)O complex has been studied for the normal and three (15)N isotopomers, leading to rotational, centrifugal distortion, and nuclear quadrupole coupling constants, the molecular structure, and a binding energy of 8.4 kJ mol(-1). Here, it is shown that many DME-N(2)O-type complexes are bound with three intermolecular bonds and that the internal rotation splitting due to the methyl groups in the rotational spectrum was fixed by complexation, implying that many weak intermolecular bonds can fix the flexible motions and maintain a rigid structure. If the model we are proposing for DME-N(2)O-type complexes can be applied to biomolecules, it may give something a clue to solve the biological riddle on the dynamic character of biomolecules that have conflicting properties of being rigid and binding weakly.