Rotational analysis of several bands in the high-resolution infrared spectrum of butadiene-1-13C1: assignment of vibrational fundamentals

Spectroscopic and Computational Characterization of 2-Aza-1,3-butadiene, a Molecule of Astrochemical Significance

Being N-substituted unsaturated species, azabutadienes are molecules of potential relevance in astrochemistry, ranging from the interstellar medium to Titan’s atmosphere. 2-Azabutadiene and butadiene share a similar conjugated π system, thus allowing investigation of the effects of heteroatom substitution. More interestingly, 2-azabutadiene can be used to proxy the abundance of interstellar butadiene. To enable future astronomical searches, the rotational spectrum of 2-azabutadiene has been investigated up to 330 GHz. The experimental work has been supported and guided by accurate computational characterization of the molecular structure, energetics, and spectroscopic properties of the two possible forms, trans and gauche. The trans species, more stable by about 7 kJ/mol than gauche-2-azabutadiene, has been experimentally observed, and its rotational and centrifugal distortion constants have been obtained with remarkable accuracy, while theoretical estimates of the spectroscopic parameters are reported for gauche-2-azabutadiene.

De novo structure determination of butadiene by isotope-resolved rotational Raman spectroscopy

Isotope-selective rotational spectroscopy allows to calculate molecular structures independent of assumptions or theoretical predictions. Here, we present the first de novo structure determination based on mass-correlated rotational Raman spectroscopy, analyzing the carbon atom positions of butadiene. Mass correlation allowed us to analyze signals of rare 13C isotopologues at natural abundance, without interference from the main isotopologue signals. Fitted rotational constants and structural parameters confirm literature data from rovibrational spectroscopy of synthetic isotopologues and electron diffraction experiments.

s-trans-1,3-Butadiene and Isotopomers: Vibrational Spectra, Scaled Quantum-Chemical Force Fields, Fermi Resonances, and C−H Bond Properties

Quadratic quantum-chemical force fields have been determined for s-trans-1,3-butadiene using B3LYP and MP2 methods. Basis sets included 6-311++G, cc-pVTZ, and aug-cc-pVTZ. Scaling of the force fields was based on frequency data for up to 11 isotopomers, some of these data being original. A total of 18 scale factors were employed, with, in addition, an alteration to one off-diagonal force constant in the A(u) species. MP2 calculations without f functions in the basis perform badly in respect of out-of-plane bending mode frequencies. Centrifugal distortion constants and harmonic contributions to vibration-rotation constants (alphas) have been calculated. Existing experimental frequency data for all isotopomers are scrutinized, and a number of reassignments and diagnoses of Fermi resonance made, particularly in the nu(CH) region. The three types of CH bond in butadiene were characterized in terms of bond length and isolated CH stretching frequency, the latter reflecting data in the nu(CD) region. Broad agreement was achieved with earlier results from local mode studies. Differences in CH bond properties resemble similar differences in propene. A simplified sample setup for recording FT-Raman spectra of gases was applied to four isotopomers of butadiene.

Equilibrium Structures for Butadiene and Ethylene: Compelling Evidence for Π-Electron Delocalization in Butadiene

Equilibrium structures have been determined for s-trans-1,3-butadiene and ethylene after adjusting the rotational constants obtained from rotational spectroscopy by vibration-rotation constants calculated from the results of quantum chemical calculations. For butadiene, the formal C=C bond length is 1.338 A, and the formal C-C bond length is 1.454 A. For ethylene, the C=C bond length is 1.3305 A. These values appear to be good to 0.001 A. It is shown for the first time that pi-electron delocalization has the structural consequences of increasing the length of the formal double bond by 0.007 A and decreasing the length of the formal single bond by 0.016 A. Comparisons are made with structures computed with several quantum chemical models. The MP2/cc-pVTZ results agree best with the new re structure.