The microwave spectrum of a two-top peptide mimetic: the N-acetyl alanine methyl ester molecule

The rotational spectrum of N-acetyl alanine methyl ester, a derivative of the biomimetic, N-acetyl alanine N'-methyl amide or alanine dipeptide, has been measured using a mini Fourier transform spectrometer between 9 and 25 GHz as part of a project undertaken to determine the conformational structures of various peptide mimetics from the torsion-rotation parameters of low-barrier methyl tops. Torsion-rotation splittings from two of the three methyl tops capping the acetyl end of the -NH-C(=O)- and the methoxy end of -C(=O)-O- groups account for most of the observed lines. In addition to the AA state, two E states have been assigned and include an AE state having a torsional barrier of 396.45(7) cm(-1) (methoxy rotor) and an EA state having a barrier of 64.96(4) cm(-1) (acetyl rotor). The observed torsional barriers and rotational constants of alanine dipeptide and its methyl ester are compared with predictions from M?ller-Plesset second-order perturbation theory (MP2) and density functional theory (DFT) in an effort to explore systematic errors at the two levels of theory. After accounting for zero-point energy differences, the torsional barriers at the MP2/cc-pVTZ level are in excellent agreement with experiment for the acetyl and methoxy groups while DFT predictions range from 8% to 80% too high or low. DFT is found to consistently overestimate the overall molecular size while MP2 methods give structures that are undersized. Structural discrepancies of similar magnitude are evident in previous DFT results of crystalline peptides.     

The ν1, ν2, and ν3 bands of carbonyl chlorofluoride (COFCl) at 5.3, 9.1, and 13.1 μm: position and intensity parameters and their use for atmospheric studies

A list of line positions and, for the first time, of line intensities was generated for the ν₁, ν₂, and ν₃ fundamental bands of the ¹²C¹⁶OF³⁵Cl and ¹²C¹⁶OF³⁷Cl isotopologs of carbonyl chlorofluoride, located at 5.3, 9.1, and 13.1 μm, respectively. In addition, for the most abundant isotopolog (¹²C¹⁶OF³⁵Cl) this linelist includes also the contributions from the first two associated hot bands. The parameters included in this database were generated by combining the results of previous experimental analyses and ab initio calculations [Perrin A, Flaud JM, Bürger H, Pawelke G, Sander S, Willner H. First high resolution analysis of the six fundamental bands ν₁, ν₂, ν₃, ν₄, ν₅ and ν₆ of COF³⁵Cl in the 340 to 2000 cm⁻¹ region. J Mol Spectrosc 2001;209:122-232; Demaison J, Perrin A, Bürger H. Ab initio anharmonic force field and equilibrium structure of carbonyl chlorofluoride. J Mol Spectrosc 2003;221:47-56]. For the purpose of the present work, a partial re-investigation of the ν₁ of COF³⁵Cl was performed, together with the first identification of the ν₂ band of COF³⁷Cl.These parameters were generated in order to improve the quality of remote sensing of the atmosphere in the mid-IR. Analyses of atmospheric solar occultation spectra measured by the JPL MkIV interferometer show that the new linelist not only improves the quality of retrievals of COFCl, but also of several other gases whose absorptions overlap those of COFCl.     

High-resolution infrared and millimeterwave spectra of the v3=1 vibrational state of NF3 at 907 cm

The ν₃^±1 perpendicular band of ¹⁴NF₃ ( cm⁻¹) has been studied with a resolution of 2.5 × 10⁻³ cm⁻¹, and 3682 infrared (IR) transitions (J_max=55, K_max=45) have been assigned. These transitions were complemented by 183 millimeterwave (MMW) rotational lines (J_max=25, K_max=19) in the 150–550 GHz region (precision 50–100 kHz). The kl=+1 level reveals a strong A₁/A₂ splitting due to the l(2,2) rotational interaction (q=−4.05 × 10⁻³ cm⁻¹) while the kl=−2 and +4 levels exhibit small A₁/A₂ splittings due to l(2,−4) and l(0,6) rotational interactions. All these splittings were observed by both experimental methods. Assuming the v₃=1 vibrational state as isolated, a Hamiltonian model of interactions in the D reduction, with l(2,−1) rotational interaction (r=−1.96 × 10⁻⁴ cm⁻¹) added, accounted for the observations. A set of 26 molecular constants reproduced the IR observations with σ_IR=0.175 × 10⁻³ cm⁻¹ and the MMW data with σ_MMW=134 kHz. The Q reduction was also performed and found of comparable quality while the QD reduction behaved poorly. This may be explained by a predicted Coriolis interaction between v₃=1 and v₁=1 (A₁, 1032.001 cm⁻¹) which induces a slow convergence of the Hamiltonian in the QD reduction but has no major influence on the other reductions. The experimental equilibrium structure could be calculated as: r_e(N–F)=1.3676 Å and (FNF)=101.84°.     

Ab initio anharmonic force field and equilibrium structure of vinyl bromide

The quadratic, cubic, and semi-diagonal quartic force field of vinyl bromide has been calculated at the MP2 level of theory employing a basis set of triple-ζ quality including a relativistic pseudopotential on bromine. A semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational interaction parameters calculated from the ab initio force field. This structure is in excellent agreement with the ab initio structure calculated at the CCSD(T) level of theory using a basis set of quadruple-ζ quality and an offset correction. The experimental mass-dependent r_m structures are also determined and their accuracy is discussed.     

Equilibrium vs ground-state planarity of the CONH linkage

Planarity of the XC(=)NHY linkage has been investigated in unprecedented detail in a number of relatively simple compounds, including formamide (X = Y = H), acetamide (X = CH3, Y = H), urea (X = NH2, Y = H), carbamic acid (X = OH, Y = H), and methyl carbamate (X = OCH3, Y = H). Reliable estimates of the equilibrium structures of formamide, cyanamide, acetamide, urea, carbamic acid, methylamine, dimethyl ether, and methyl carbamate are derived, mostly for the first time. It is shown that formamide, considered prototypical for the amide linkage, is not typical as it has a planar equilibrium amide linkage corresponding to a single-minimum inversion potential around N. In contrast, several molecules containing the CONH linkage seem to have a pyramidalized nitrogen at equilibrium and a double-minimum inversion potential with a very small inversion barrier allowing for an effectively planar ground-state structure. Observables of rotational spectroscopy, including ground-state inertial defects, quadrupole coupling and centrifugal distortion constants, and dipole moment components, as well as equilibrium C=O and C-N bond lengths are reviewed in their ability to indicate the planarity of the effective and possibly the equilibrium structures.     

The millimeterwave spectrum of germyl fluoride: Determination and comparison of the effective, substitution, and equilibrium structures

The ground state rotational spectrum of germyl fluoride was measured up to 1273 GHz (J ≤ 63). The rotational constants and quartic and sextic centrifugal distortion constants have been determined accurately for five isotopic species in natural abundance (^{70/72/73/74/76}Ge). The high accuracy of the rotational constants of these five isotopomers allowed us to study the mass dependence of the substitution coordinate of Ge. Equilibrium rotational constants of ⁷⁴GeH₃F were deduced with the help of the axial rotational constant and the rotation-vibration interaction constants determined by high resolution infrared spectroscopy. The r₀, r_,I, and r_e structures of GeH₃F were determined.     

Rotational Spectra of CH2DCCH and CH3CCD: Experimental and ab Initio Equilibrium Structures of Propyne

The ground state rotational spectra of CH₂DCCH and CH₃CCD (main species and ¹³C-substituted species) have been measured up to 470 GHz. Accurate rotational and centrifugal distortion constants have been determined. r₀, r_s, r_ε,I, and r^ρ_m, structures of propyne have been calculated. The ab initio structure has also been calculated using three different methods (SCF, MP2, and QCISD) and two basis sets (DZP and TZ2P). Offsets have been derived empirically using molecules containing structural units present in propyne and whose equilibrium structures have been determined previously. A near-equilibrium structure has been estimated to be acetylenic r(C---H) = 1.061 (1) Å, r(CC) = 1.204 (1) Å, r(C---C) = l.458 (2) Å, methyl r(C---H) = 1.089 (1) Å, and (CCH) = 110.7 (5)°.     

Centrifugal distortion and internal rotation analysis of the rotational spectra of N-methylmethanimine d0 and d5

The ground state millimeter-wave spectra of CH₃NCH₂ and CD₃NCD₂ have been measured. The rotational constants, centrifugal distortion constants, and barrier hindering internal rotation of the methyl group have been determined for both species. For the parent species I_α and ?(i,a) were also obtained, and for the perdeuteriated species the quadrupole coupling constants of ¹⁴N were determined.     

Equilibrium structure and spectroscopic constants of maleic anhydride

The quadratic, cubic, and semi-diagonal quartic force fields of maleic anhydride have been calculated at the MP2 level of theory employing the cc-pVTZ basis set. The spectroscopic constants derived from the force field are in excellent agreement with the corresponding experimental values. The semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational corrections calculated from the cubic force field. This semi-experimental equilibrium structure is in excellent agreement with the ab initio structures computed at the CCSD(T) level of theory and it is closer to the ab initio structure than the purely experimental (or empirical) structures r ₀, r _m⁽¹⁾, and r _m⁽²⁾ obtained by microwave spectroscopy as well as the equilibrium structure derived from gas-phase electron diffraction data.