The pure rotational spectrum of pyrimidine (m-C4H4N2), the meta-substituted dinitrogen analog of benzene, has been studied in the millimeter-wave region from 235 GHz to 360 GHz. The rotational spectrum of the ground vibrational state has been assigned and fit to yield accurate rotational and distortion constants. Over 1700 distinct transitions were identified for the normal isotopologue in its ground vibrational state and least-squares fit to a partial sextic S-reduced Hamiltonian. Transitions for all four singly substituted 13C and 15N isotopologues were observed at natural abundance and were likewise fit. Deuterium-enriched samples of pyrimidine were synthesized, giving access to all eleven possible deuterium-substituted isotopologues, ten of which were previously unreported. Experimental values of rotational constants and computed values of vibration–rotation interaction constants and electron-mass corrections were used to determine semi-experimental equilibrium structures (reSE) of pyrimidine. The reSE structure obtained using coupled-cluster with single, double, and perturbative triple excitations [CCSD(T)] corrections shows exceptional agreement with the re structure computed at the CCSD(T)/cc-pCV5Z level (≤0.0002 Å in bond distance and ≤0.03° in bond angle). Of the various computational methods examined, CCSD(T)/cc-pCV5Z is the only method for which the computed value of each geometric parameter lies within the statistical experimental uncertainty (2σ) of the corresponding semi-experimental coordinate. The exceptionally high accuracy and precision of the structure determination is a consequence of the large number of isotopologues measured, the precision and extent of the experimental frequency measurements, and the sophisticated theoretical treatment of the effects of vibration–rotation coupling and electron mass. Taken together, these demanding experimental and computational studies establish the capabilities of modern structural analysis for a prototypical monocyclic aromatic compound.

Iso-polyhalomethanes are known reactive intermediates that play a pivotal role in the photochemistry of halomethanes in condensed phases. In this work, iso-bromoform (iso-CHBr(3)) and its deuterated isotopomer were characterized by matrix isolation infrared and UV/visible spectroscopy, supported by ab initio and density functional theory calculations, to further probe the structure, spectroscopy, and photochemistry of this important intermediate. Selected wavelength laser irradiation of CHBr(3) isolated in Ar or Ne matrices at ~5 K yielded iso-CHBr(3); the observed infrared and UV/visible absorptions are in excellent agreement with computational predictions, and the energies of various stationary points on the CHBr(3) potential energy surface were characterized computationally using high-level methods in combination with correlation consistent basis sets. These calculations show that, while the corresponding minima lie ~200 kJ/mol above the global CHBr(3) minimum, the isomer is bound by some 60 kJ/mol in the gas phase with respect to the CHBr(2) + Br asymptote. The photochemistry of iso-CHBr(3) was investigated by selected wavelength laser irradiation into the intense S(0) → S(3) transition, which resulted in back photoisomerization to CHBr(3). Intrinsic reaction coordinate calculations confirmed the existence of a first-order saddle point connecting the two isomers, which lies energetically below the threshold of the radical channel. Subsequently, natural bond orbital analysis and natural resonance theory were used to characterize the important resonance structures of the isomer and related stationary points, which demonstrate that the isomerization transition state represents a crossover from dominantly covalent to dominantly ionic bonding. In condensed phases, the ion-pair dominated isomerization transition state structure is preferentially stabilized, so that the barrier to isomerization is lowered.

Advances in software and hardware have promoted the use of computational chemistry in all branches of chemical research to probe important chemical concepts and to support experimentation. Consequently, it has become imperative that students in the modern undergraduate curriculum become adept at performing simple calculations using computational software, interpreting computational data, and applying computational data to explain chemical phenomena. We utilize computational chemistry in a high-enrollment (>1200 students/year), undergraduate organic chemistry laboratory course in a manner similar to that of organic chemistry researchers. We have employed WebMO as a webbased, easy-to-use, and free front-end interface for Gaussian09 that allows our students to complete ab initio and density functional theory (DFT) calculations throughout the curriculum. Rather than an isolated exposure to computational chemistry, our students use computational chemistry to obtain a deeper understanding of their experimental work throughout the entire semester. By integrating calculations into the curriculum, the focus moves away from performing the calculations to providing insight into chemical phenomena and understanding experimental results. We provide here both an overview of the introductory laboratory experiment and our integrated approach.

The rotational spectrum of thiophene (c-C4H4S) has been collected between 8 and 360 GHz. Samples of varying deuterium-enrichment were synthesized to yield all possible deuterium-substituted isotopologues of thiophene. A total of 26 isotopologues have been measured and least-squares fit using A- and S-reduced distorted-rotor Hamiltonians in the Ir representation. The resultant rotational constants (A0, B0, and C0) from each reduction were converted to determinable constants (A″, B″, and C″) to remove the impact of centrifugal distortion. The computed vibrational and electron mass corrections [CCSD(T)/cc-pCVTZ] were applied to the determinable constants to obtain semi-experimental equilibrium rotational constants (Ae, Be, and Ce) for 24 isotopologues. A precise semi-experimental equilibrium (reSE) structure has been achieved from a least-squares fit of the equilibrium moments of inertia. The combination of the expanded isotopologue rotational data with high-level computational work establishes a precise reSE structure for this sulfur-containing heterocycle. The CCSD(T)/cc-pCV5Z structure has been obtained and corrected for the extrapolation to the complete basis set, electron correlation beyond CCSD(T), relativistic effects, and the diagonal Born–Oppenheimer correction. The precise reSE structure is compared to the resulting “best theoretical estimate” structure. Several of the best theoretical re structural parameters fall within the narrow statistical limits (2σ) of the reSE results. The possible origin of the discrepancies for the computed parameters that fall outside the statistical uncertainties is discussed.

The rotational spectrum of pyridazine (o-C4H4N2), the ortho disubstituted nitrogen analog of benzene, has been measured and analyzed in the gas phase. For the ground vibrational state of the normal isotopolog, over 2000 individual rotational transitions have been identified between 238 and 360 GHz and have been fit to 13 parameters of a 6th-order centrifugal distortion Hamiltonian. All transitions in this frequency region can now be predicted from this model to near experimental accuracy, i.e., well enough for the purpose of any future radio-astronomical search for this species. Three isotopologs, [3-(13)C]-C4H4N2, [4-(13)C]-C4H4N2, and [1-(15)N]-C4H4N2, have been detected in natural abundance, and several hundred lines have been measured for each of these species and fit to 6th-order Hamiltonians. Ten additional isotopologs were synthesized with enhanced deuterium substitution and analyzed to allow for a complete structure determination. The equilibrium structure (Re) of pyridazine was obtained by correcting the experimental rotational constants for the effects of vibration-rotation coupling using interaction constants predicted from CCSD(T) calculations with an ANO0 basis set and further correcting for the effect of electron mass. The final Re structural parameters are determined with excellent accuracy, as evidenced by their ability to predict 28 independent moments of inertia (Ia and Ib for 14 isotopologs) very well from 9 structural parameters. The rotational spectra of the six lowest-energy fundamental vibrational satellites of the main isotopolog have been detected. The rotational spectra of the five lowest-energy vibrational satellites have been assigned and fit to yield accurate rotational and distortion constants, while the fit and assignment for the sixth is less complete. The resultant vibration-rotation interaction (α) constants are found to be in excellent agreement with ones predicted from coupled-cluster calculations, which proved to be the key to unambiguous assignment of the satellite spectra to specific vibration modes.