Methacrylonitrile is believed to be an astrochemically relevant molecule due to the detection of acrylonitrile (C₃H₃N) in Saturn’s moon, Titan, and in the interstellar medium. We synthesized methacrylonitrile via the hydrocyanation and subsequent dehydration of acetone and obtained its rotational spectrum in the 130 – 360 GHz frequency region. The main isotopologue ground state has been least-squares fit to a sextic Hamiltonian accounting for internal rotation splitting from the 3-fold symmetric methyl rotor, and resulting spectroscopic constants compare well with those previously reported. The improved determination of centrifugal distortion constants improve the spectral prediction over a broad frequency range and thus the radioastronomical search for this molecule. Additionally, a study of pyrimidine, using 16 isotopologues, demonstrated that it is possible to determine very accurate semi-experimental equilibrium structures (r_e^SE) using multiple isotopic substitution combined with coupled-cluster calculations treating the vibration-rotation interaction and the electron mass correction. With the goal of such a structure determination for methacrylonitrile, we have analyzed the spectra of all singly-substituted heavy-atom isotopologues (¹³C and ¹⁵N), which were detectable at natural abundance, and least-squares fit them to sextic Hamiltonians. To obtain deuterated isotopologues, the synthesis of methacrylonitrile was modified by using partially deuterated or fully deuterated acetone to yield samples of varying deuterium incorporation. We have currently assigned the rotational spectra of 22 isotopologues, with additional isotopologues yet to be analyzed. The current r_e^SE will be presented and compared to theoretical r_e structures.

1H-1,2,3-triazole (C_s; κ = 0.94; μ_a = 4.1 D, μ_b = 1.6 D) and 2H-1,2,3-triazole (C_2v; κ = 0.82; μ_b = 0.52 D) are tautomers (C₂H₃N₃), the former of which is formally the product of a 1,3-dipolar cycloaddition of two explosive compounds, hydrazoic acid and acetylene. We have analyzed the rotational spectra of these tautomers in the 130 – 360 GHz frequency range and least-squares fit the observed transitions to sextic Hamiltonians with low error (σ < 50 kHz). While ¹³C and ¹⁵N isotopologues have been observed at natural abundance and similarly analyzed, several synthetic techniques had to be employed to access deuterium-substituted isotopologues. The rotational constants of the numerous isotopologues, along with vibration-rotation interaction and electron mass corrections predicted using coupled-cluster and density functional theories, have been used to determine highly precise semi-experimental equilibrium structures (r_e^SE) of the 1,2,3-triazoles. Equilibrium structures calculated at the above-mentioned levels of theory, as well as a computational prediction using basis set extrapolation and additional corrections for the 2H structure, are compared to the semi-experimental structures.

This is a continued discussion regarding the substitution effectcts on the three-dimensional shape and electron distribution of three methylamine substituted pyridines, 2-, 3-, and 4-picolylamine. The microwave spectra of all three molecules were collected over the frequency range of 4-18 GHz. In addition, the ¹³C and ¹⁵N isotopologues, along with the deuterated amine groups, were collected for each molecule in natural abundance. From these spectra, exact structures were determined which allowed for accurate rotation of the quadruple constants from the molecular frame into an internal axis. Next, an extended Townes and Dailey analysis was used to determine the lone pair density around each nitrogen and then compared to those of benzylamine and pyridine. Results of this analysis and how it explains the configuration of the methylamine group in each of the picolylamines will be discussed.

The rotational spectrum of oxazole (c-C₃H₃NO, C_s), an astrochemically relevant aromatic heterocycle, has been studied from 40 GHz to 360 GHz. Transitions of the ground vibrational state of the main isotopologue, as well as those of the five heavy-atom isotopologues (¹⁵N, ¹⁸O, and three ¹³C), were detectable at natural abundance and have been fit to sextic centrifugally distorted Hamiltonians. All species are well fit with statistical uncertainties less than 40 kHz. Precise rotational and centrifugal distortion constants obtained for these species are compared to those obtained via B3LYP and CCSD(T) calculations. The experimental rotational constants have been corrected for vibration-rotation interactions and electron mass distributions. Preparation of deuterated isotopologues is underway and these data should yield a highly precise, semi-experimental equilibrium (r_e^SE) structure of oxazole, on par with the most precisely determined molecular structures.

Thiazole (C₃H₃NS, C_s, μ_a = 1.286 D, μ_b = 0.966 D) is a five-membered aromatic heterocycle containing a 1,3-substituted sulfur and nitrogen. We analyzed the rotational spectra of thiazole and twenty-one of its isotopologues from 130 – 360 GHz. Heavy atom ¹³C, ³⁴S, ³³S, and ¹⁵N isotopologues were observable in the rotational spectrum of the normal isotopologue at their natural abundance. Two syntheses were performed to generate a variety of deuterium-substituted isotopologues, resulting in multiple isotopic substitutions of each atom in the molecule. The resultant determinable rotational constants were computationally corrected for vibration-rotation interactions and electron mass with CCSD(T) calculations and 22 total isotopologues were least-squares fit to afford the semi-experimental equilibrium structure (r_e^SE). Theoretical structures were computed at several levels of theory up to CCSD(T)/cc-pCV5Z. The quintuple zeta structure was further refined to account for extrapolation to the complete basis set limit, residual electron correlation beyond CCSD(T), relativistic effects, and the diagonal Born-Oppenheimer correction. The resultant r_e^SE structure and best theoretical structure are compared.

l0pt Figure We are continuing our project of obtaining high resolution vibration-rotation spectra of propane isotopologues using the Far-IR beamline at the Canadian National Synchrotron (CLS). We have already reported on all the singly ¹³C and singly D substituted varieties as well as the 2,2-D₂ species at previous ISMS meetings (2017-19) and in a recent paper on 2-¹³C-Propane Daunt, Grzywacz, Western, Lafferty, Flaud, Billinghurst, and Hutchings, J. Mol. Structure, in press (doi:10.1016/j.molstruc.2020.127851). These studies have allowed us to determine ground state inertial and centrifugal distortion rotational constants for these molecules that have no pure rotational spectra in the literature except for the 6 lines reported by LideD. R. Lide, J. Chem. Physics 33, 1514-1518 (1960).n 1960 of normal propane and two ¹³C and D versions. In this talk we will present the high resolution spectra (0.00096 cm⁻¹) for the CCC skeletal bendings of CH₃CH₂CD₃, CD₃CH₂CD₃ and C₃D₈. These studies have yielded corrected observed band originsGayles and King, Spectrochim. Acta 21, 543-557 (1965); K. M. Gough, W. F. Murphy and K. Raghavachari, J. Chem. Phys. 87, 3332-3340 (1987).nd rotational constants for the three species. Preliminary values for each species are listed here in wavenumbers. CH₃CH₂CD₃: ν₉ = 335.664740(40), A₀ = 0.8185513(12), B₀ = 0.24400666(39), C₀ = 0.21852642(41); CD₃CH₂CD₃: ν₁₆ = 306.4, A₀ = 0.711202, B₀ = 0.213021, C₀ = 0.193244; C₃D₈: ν₉ = 303.936065(23), A₀ = 0.58742224(42), B₀ = 0.20872437(23), C₀ = 0.18588200(18).

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Footnotes:

Daunt, Grzywacz, Western, Lafferty, Flaud, Billinghurst, and Hutchings, J. Mol. Structure, in press (doi:10.1016/j.molstruc.2020.127851).. D. R. Lide, J. Chem. Physics 33, 1514-1518 (1960).i Gayles and King, Spectrochim. Acta 21, 543-557 (1965); K. M. Gough, W. F. Murphy and K. Raghavachari, J. Chem. Phys. 87, 3332-3340 (1987).a

Thiophene (C₄H₄S, C_2v symmetry, μ_a = 0.55 D) is the sulfur analog of furan. With the intent of improving its gas-phase structure determination, its rotational spectrum was collected from 8 – 360 GHz, and 21 deuterium containing isotopologues were synthesized and their rotational spectra were collected from 130 – 360 GHz. The heavy atom ¹³C, ³⁴S, and ³³S isotopologues were observable in the rotational spectra of the normal isotopologue and several deuterium containing forms at natural abundance. The resultant determinable rotational constants (A^′′, B^′′, C^′′) were computationally corrected for vibration-rotation interactions and electron mass with CCSD(T) calculations and 24 total isotopologues were least-squares fit to afford the semi-experimental equilibrium structure (r_e^SE). For comparison, theoretical structures were determined at several levels of theory up to CCSD(T)/cc-pCV5Z. The quintuple zeta structure was further refined to account for extrapolation to the complete basis set limit, residual electron correlation beyond CCSD(T), relativistic effects, and the diagonal Born-Oppenheimer correction. The resultant r_e^SE structure and "best" theoretical structure are compared.

We obtained an improved semi-experimental equilibrium structure (r_e^SE) of pyridazine (o-C₄H₄N₂) by improving the distorted-rotor least-squares fits of previously reported experimental data by incorporating fits of four previously unobserved isotopologues and by using higher level, isotopologue-dependent ab initio corrections from CCSD(T)/cc-pCVTZ in the r_e^SE determination. As a result, we improved the determination of the parameters of pyridazine by a factor of two, obtaining 2σ uncertainties of less than 0.001 Å and 0.04^° in the bond distances and angles, respectively. Additionally, we have obtained an improved theoretical prediction of the structure by using a CCSD(T)/cc-pCV5Z optimization and applying corrections to account for extrapolation to the complete basis set limit, for effects of electron correlation, for relativistic effects, and for the Born-Oppenheimer approximation. The resulting theoretical structural parameters are almost all within the corresponding uncertainties of the improved r_e^SE parameters. Finally, we analyzed the effect of including additional isotopologues on the r_e^SE structure and extended the analysis for comparison to the minimal set of isotopologues as used in substitution structure determinations.

Only a few molecules with two iodine atoms have been studied using rotational spectroscopy probably due to the complex hyperfine splitting structure arising from the presence of two iodine nuclei. The high resolution rotational spectroscopic observation of 1,1-diiodoethane is investigated using a pulsed jet, cavity Fourier transform microwave (FTMW) spectrometer over the frequency range 11.5–18 GHz for the first time. The rotational constants, the centrifugal distortion constants, the nuclear spin-rotation coupling constants, and the complete tensor components of the nuclear quadrupole coupling for both iodine nuclei have been determined and reported. The fitted rotational constants are A = 4548.320446(47), B = 625.629141(55), C = 558.798939(43) MHz and the nuclear quadrupole coupling constants are χ_aa = -1089.8125(7), χ_bb – χ_cc = -542.3162(13), \lvertχ_ab\rvert = 1215.7505(10), χ_bc = 340.8983(14), and \lvert χ_ac\rvert = 562.4206(19) MHz. No A-E splittings due to the methyl group internal rotation were observed. Many dipole-forbidden/electric quadrupole coupling allowed transitions were observed in the spectrum due to the large iodine quadrupole coupling effect. Quantum chemical calculations were performed at the CCSD(T)/aug-cc-pVTZ-pp level of theory. The calculated rotational constants, centrifugal distortion constants, and hyperfine constants were used to guide the data analysis.

In preparation for our FTMW study of the complex between hydrogen and silver bromide, H₂-AgBr, we have investigated the microwave spectra of our source of bromine, dibromodifluoromethane, CBr₂F₂. Previous sources of bromine to produce AgBr proved either too damaging to the instrumentation in the long term (Br₂), or did not produce enough AgBr to be useful (CHBr₃). In addition, silver bromide, originally studied by Hoeft, Lovas, Tiemann, and Torring in 1971,J. Hoeft, F. J. Lovas, E. Tiemann, T. Torring, Z. Naturforsch. 25a, 35 (1970).nd remeasured by Evans and Gerry in 2000,C. J. Evans, M. C. L. Gerry, J. Chem. Phys. 112, 1321 (2000).as remeasured once more. We plan to produce H₂-AgBr, using laser ablation of a silver rod within a supersonic expansion of an argon carrier gas containing H₂ and CBr₂F₂. This is part of our ongoing study of hydrogen complexed with metal halides which include H₂-CuF,D. J. Frohman, G. S. Grubbs II, Z. Yu, S. E. Novick, Inorg. Chem. 52, 816 (2013).₂-AgCl,G. S. Grubbs II, D. A. Obenchain, H. M. Pickett, S. E. Novick, J. Chem. Phys. 141, 114306 (2014).nd H₂-AuCl.D. A. Obenchain, G. S. Grubbs II, H. M. Pickett, S. E. Novick, J. Chem. Phys. 146, 204302 (2017).hese molecules are “models” of the molecular hydrogen storage in the cavities of metal organic frameworks (MOFs).

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Footnotes:

J. Hoeft, F. J. Lovas, E. Tiemann, T. Torring, Z. Naturforsch. 25a, 35 (1970).a C. J. Evans, M. C. L. Gerry, J. Chem. Phys. 112, 1321 (2000).w D. J. Frohman, G. S. Grubbs II, Z. Yu, S. E. Novick, Inorg. Chem. 52, 816 (2013).H G. S. Grubbs II, D. A. Obenchain, H. M. Pickett, S. E. Novick, J. Chem. Phys. 141, 114306 (2014).a D. A. Obenchain, G. S. Grubbs II, H. M. Pickett, S. E. Novick, J. Chem. Phys. 146, 204302 (2017).T

Pure rotational spectra of the ³⁷ClZnCH₃, ClZnCD₃, and ClZn¹³CH₃ isotopologues of monomeric ClZnCH₃ (~X¹A₁) have been recorded using millimeter-wave direct absorption techniques in the frequency range 263 – 303 GHz. These species were synthesized in the gas phase in a DC discharge by the reaction of zinc vapor, produced in a Broida-type oven, with ³⁷ClCH₃ (in natural chlorine abundance), ClCD₃, or Cl¹³CH₃. The data for each isotopologue were analyzed with a symmetric top Hamiltonian and rotational and centrifugal constants determined. In combination with previous measurements of Cl⁶⁴ZnCH₃, Cl⁶⁶ZnCH₃, and Cl⁶⁸ZnCH₃, an r_m⁽²⁾ structure was determined for this organozinc compound. The bond lengths were calculated to be r_Cl−Zn = 2.0831(1) Å, r_Zn−C = 1.9085(1) Å, and r_C−H = 1.1806(5) Å. The H-C-H bond angle was found to be 110.5^° – slightly larger than that in methane. These data serve to benchmark future structure calculations of organozinc compounds, which are widely used in organic synthesis.

Contrary to the common sense that the C-H bond-length is longer than that of the C-D bond due to the anharmonicity of the potential, Baba group found, from high-resolution laser spectroscopy, that ro-vibrationally averaged bond lengths of C-H and C-D are observed as being almost identical (r_0,eff(C-H) ≅ r_0,eff(C-D)) for planar aromatic hydrocarbons, such as benzeneS. Kunishige, M. Baba, et al., J. Chem. Phys. 143, 244302 (2015). naphthaleneM. Baba, U. Nagashima, et al., J. Chem. Phys. 135, 054305 (2011). and anthraceneM. Baba, U. Nagashima, et al., J. Chem. Phys. 130, 134315 (2009). The vibrationally averaged bond lengths of benzene deduced from experimental rotational constants and ab initio geometry^b are r₀(C-H) = r₀(C-D) = 1.0831 Å.^a In this study, the vibrationally averaged structures of benzene are derived from the DVR (Discrete Variable Representation) wavefunction based on the 3D potential energy surface at the level of valence-CCSD(T)/[aVQZ(H,C)] theory. The zero-point C-H(D) bond-length, r_0,eff(C-H(D)), determined from the experimental effective rotational constants, is, in definition, the r₀ bond-length projected onto the a-b principal axis plane, r_0,proj(C-H(D)). The DVR wavefunction gives r_0,proj(C-H) = 1.0815 Å and r_0,proj(C-D) = 1.0819 Å, so that the difference is of a negligibly small value −0.0004 Å. Thus, the seemingly strange experimental finding (r_0,eff(C-H) ≅ r_0,eff(C-D)), which is controversial to the common sense, is resolved. The details will be given in the next presentation (part II.) S. Kunishige, M. Baba, et al., J. Chem. Phys. 143, 244302 (2015)., M. Baba, U. Nagashima, et al., J. Chem. Phys. 135, 054305 (2011)., M. Baba, U. Nagashima, et al., J. Chem. Phys. 130, 134315 (2009)..

Since the 30 dimensional potential energy surface is beyond our reach, we tried to disclose why the C-H and C-D bond-lengths are observed as being almost identicalS. Kunishige, M. Baba, et al., J. Chem. Phys. 143, 244302 (2015).n terms of C-H stretching (L_str), out-of-plane (L_⊥), and in-plane (L_||) local modes with respect to the C_α-H(D)_α bond in the virtual triatomic molecules [C₅H(D)₅]-[C_α]-H(D)_α. The potential energy surface was determined at the valence-CCSD(T)/[aVQZ (H,C)] level of theory, and r₀-structure was determined from the DVR3D wavefuctions in Discrete Variable Representation. The virtual [C₅H(D)₅]-[C_α]-H(D)_α molecule has its energy minimum at the linear configuration, so that our theory for linear triatomicsT. Hirano, U. Nagashima, P. Jensen, J. Mol. Spectrosc. 343, 54 (2018); T. Hirano, U. Nagashima, M. Baba, J. Mol. Spectrosc. 369, 111252 (2020); and references therein.an be applied. The C_α-H(D)_α stretching local mode (L_str) gives, as usual, longer C_α-H_α than C_α-D_α bond-lengths due to its anharmonicity. However, in both L_⊥ and L_|| modes, the vibrationally averaged bond-length projected onto the principal axis is shorter for C_α-H_α than for C_α-D_α due to the larger averaged bending angle for the former bond. When we consider bond-lengths projected onto the a-b principal axis plane, r_0,proj, these antithetical factors, i.e., one in the L_str mode against the others in L_⊥ and L_|| modes, nearly cancel (∆ = r(C-H) − r(C-D) = −0.0004 Å), resulting in almost the same C-H and C-D bond lengths as is experimentally reported. The vibrationally averaged structure of benzene in the zero-point vibration state is predicted to be planar, but non-flat in the peripheral C-H bonds moiety, which is confirmed from the theoretical and experimental values of the inertial defect.

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Footnotes:

S. Kunishige, M. Baba, et al., J. Chem. Phys. 143, 244302 (2015).i T. Hirano, U. Nagashima, P. Jensen, J. Mol. Spectrosc. 343, 54 (2018); T. Hirano, U. Nagashima, M. Baba, J. Mol. Spectrosc. 369, 111252 (2020); and references therein.c

A new data set of accurate semi-experimental equilibrium strctures (r_SE) for small open-shell species will be presented. In the semi-experimental approach, equilibrim rotational constants (B_e) of different isotopologues are used in a least-square-fit to retrieve the equilibirum parameters, thus implying the validity of the Born-Oppenhaimer approximation. The required B_e are obtained by correcting experimental rotational constants of the vibrational ground state (B₀) for the computed vibrational (∆B₀) and electronic (∆B_g) contributions. The latter have to be computed using ab initio methods and this imposes several constrains on the theoretical methodologies that can be employed. The ∆B_g's have been retrived from HF/aug-cc-pVTZ computations of the g-tensor and the ∆B₀'s have been obtained at the either UCCSD(T)/cc-pVTZ or the UB2PLYP-D3/cc-pVTZ level of theory, according to the level of spin-contamination. The new data set includes several radicals for which experimental rotational constants of different isotopologues have already been reported in the literature, such as CCS, C_nH with n = 1-7, NH₂, CCP, CH₂, PH₂, H₂CCN, CP, CN and more. This collection of equilibrium structures has three ultimate purposes: (i) to serve as benchmark-reference for electronic structure methods, (ii) give new insights, based on experimental data, on the molecular structure of unstable species and on how these structures changes across a group of the periodic table and (iii) to predict the rotational spectrum of less abundant isotopologues. The semi-experimental approach leads to r_SE that can have an accuracy below 0.1 mÅ for bond lenghts and 0.01^° for angles. For example, in the case of CCS radical, the C-S bond is predicted to be 1.31349(2) Å while the C-C bond lenght is found to be 1.56379(1) Å. For angles, the NH₂ molecule can be used as reference, with N-H bond distance of 1.023618(2) Å and an HNH angle of 103.1488(3)^°.