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The gas-phase rotational spectrum from 8 to 750 GHz and the high-resolution infrared (IR) spectrum of pyridazine (o-C4H4N2) have been analyzed for the ground and four lowest-energy vibrationally excited states. A combined global fit of the rotational and IR data has been obtained using a sextic, centrifugally distorted-rotor Hamiltonian with Coriolis coupling between appropriate states. Coriolis coupling has been addressed in the two lowest-energy coupled dyads (ν16, ν13 and ν24, ν9). Utilizing the Coriolis coupling between the vibrational states of each dyad and the analysis of the IR spectrum for ν16 and ν9, we have determined precise band origins for each of these fundamental states: ν16 (B1) = 361.213 292 7 (17) cm−1, ν13 (A2) = 361.284 082 4 (17) cm−1, ν24 (B2) = 618.969 096 (26) cm−1, and ν9 (A1) = 664.723 378 4 (27) cm−1. Notably, the energy separation in the ν16-ν13 Coriolis-coupled dyad is one of the smallest spectroscopically measured energy separations between vibrational states: 2122.222 (72) MHz or 0.070 789 7 (24) cm−1. Despite ν13 being IR inactive and ν24 having an impractically low-intensity IR intensity, the band origins of all four vibrational states were measured, showcasing the power of combining the data provided by millimeter-wave and high-resolution IR spectra. Additionally, the spectra of pyridazine-dx isotopologues generated for a previous semi-experimental equilibrium structure (reSE) determination allowed us to analyze the two lowest-energy vibrational states of pyridazine for all nine pyridazine-dx isotopologues. Coriolis-coupling terms have been measured for analogous vibrational states across seven isotopologues, both enabling their comparison and providing a new benchmark for computational chemistry. [ABSTRACT FROM AUTHOR]

The acid-catalyzed dehydration of regioisomeric methylcyclohexanols is a classic organic chemistry experiment featured in a variety of laboratory textbooks and literature. The mechanistic details of this reaction have received an inconsistent and occasionally inaccurate treatment, wherein the reaction has been described as a mix of E1, E2-like, acid-catalyzed E2, and base-catalyzed E2 processes. We provide an overview of how this reaction has been represented in the chemistry education literature and describe a model for its mechanistic interpretation as an E1 reaction that is typically analyzed before thermodynamic equilibrium is established. This model is supported by new experimental data and computational explorations of the reactive intermediates and cycloalkene products. We provide clear recommendations for how this reaction should be presented to students.

The affordances given to a structured, timed, and proctored paper exam are not as readily applicable in a digital medium. Accordingly, the rapid shift from in-person to online enactments may have forced instructors to consider changing their assessment practices and priorities. As assessments convey strong implicit messages about "what counts" in a given learning environment, altering what is assessed may have a profound impact on what students view as important in a course. Our four-instructor team sought to examine whether we were able to maintain emphasis on assessing "how" and "why" chemical phenomena occur online while minimizing negative impacts to students, teaching assistants, and ourselves. To support claims regarding the degree to which online assessments emphasized sensemaking relative to past exams, we characterized all summative assessments given in organic chemistry II enactments from 2016 to the present using the three-dimensional learning assessment protocol. To examine the impact of enrolling in a rapidly assembled online organic course on student outcomes, we examined the distribution of students who performed above, at, or below the final exam score predicted by their midterm performance and compared this distribution with historic norms. Results suggest that we were able to maintain emphasis on student sensemaking as our course moved online (~50% of points on exams administered remotely were dedicated to 3D performances). Additionally, the distribution of students enrolled this past spring who scored above, at, or below the final exam score predicted by their midterm performance was in line with historic norms. When taken in aggregate, our analyses suggest that organic chemistry-enrolled students maintained their ability to make sense of chemical phenomena after the pivot to online instruction. Consistent emphasis on assessing 3D learning online was achieved without adding appreciably to the burden on instructors or teaching assistants due to our assessment writing practices, streamlined approach to online grading, and pre-existing course resources. Instructional implications for assessment design, enacting team grading, and tracking student trajectories are provided in addition to a suite of assessment items with the potential to engage students in sensemaking.

A set of inexpensive and pedagogically rich experiments focusing on S[subscript N]1, E1, and E2 reactions have been updated to include modern computational and spectroscopic analyses. The S[subscript N]1 experiment involves treatment of "tert"-amyl alcohol with hydrochloric acid to generate the corresponding alkyl chloride, which is used as the starting material for the subsequent E2 reaction. The E1 experiment involves reaction of "tert"-amyl alcohol with sulfuric acid. Both elimination reactions generate a pair of isomeric methylbutenes, with the ratio being dependent on the reaction mechanism. The combination of computational chemistry and a recently developed fast heteronuclear single-quantum correlation spectroscopy method (ASAP-HSQC), in tandem with [superscript 1]H and [superscript 13]C NMR and GC-MS analysis, allows the E1 and E2 product mixtures to be fully analyzed by students.

The millimeter-wave rotational spectrum of ketene (H2C=C=O) has been collected and analyzed from 130 to 750 GHz, providing highly precise spectroscopic constants from a sextic, S-reduced Hamiltonian in the Ir representation. The chemical synthesis of deuteriated samples allowed spectroscopic measurements of five previously unstudied ketene isotopologues. Combined with previous work, these data provide a new, highly precise, and accurate semi-experimental (reSE) structure for ketene from 32 independent moments of inertia. This reSE structure was determined with the experimental rotational constants of each available isotopologue, together with computed vibration–rotation interaction and electron-mass distribution corrections from coupled-cluster calculations with single, double, and perturbative triple excitations [CCSD(T)/cc-pCVTZ]. The 2σ uncertainties of the reSE parameters are ≤0.0007 Å and 0.014° for the bond distances and angle, respectively. Only S-reduced spectroscopic constants were used in the structure determination due to a breakdown in the A-reduction of the Hamiltonian for the highly prolate ketene species. All four reSE structural parameters agree with the "best theoretical estimate" (BTE) values, which are derived from a high-level computed re structure [CCSD(T)/cc-pCV6Z] with corrections for the use of a finite basis set, the incomplete treatment of electron correlation, relativistic effects, and the diagonal Born–Oppenheimer breakdown. In each case, the computed value of the geometric parameter lies within the statistical experimental uncertainty (2σ) of the corresponding semi-experimental coordinate. The discrepancies between the BTE structure and the reSE structure are 0.0003, 0.0000, and 0.0004 Å for rC–C, rC–H, and rC–O, respectively, and 0.009° for θC–C–H. [ABSTRACT FROM AUTHOR]

In this work, we present the spectral analysis of 1H- and 2H-1,2,3-triazole vibrationally excited states alongside provisional and practical computational predictions of the excited-state quartic centrifugal distortion constants. The low-energy fundamental vibrational states of 1H-1,2,3-triazole and five of its deuteriated isotopologues ([1-2H]-, [4-2H]-, [5-2H]-, [4,5-2H]-, and [1,4,5-2H]-1H-1,2,3-triazole), as well as those of 2H-1,2,3-triazole and five of its deuteriated isotopologues ([2-2H]-, [4-2H]-, [2,4-2H]-, [4,5-2H]-, and [2,4,5-2H]-2H-1,2,3-triazole), are studied using millimeter-wave spectroscopy in the 130–375 GHz frequency region. The normal and [2-2H]-isotopologues of 2H-1,2,3-triazole are also analyzed using high-resolution infrared spectroscopy, determining the precise energies of three of their low-energy fundamental states. The resulting spectroscopic constants for each of the vibrationally excited states are reported for the first time. Coupled-cluster vibration–rotation interaction constants are compared with each of their experimentally determined values, often showing agreement within 500 kHz. Newly available coupled-cluster predictions of the excited-state quartic centrifugal distortion constants based on fourth-order vibrational perturbation theory are benchmarked using a large number of the 1,2,3-triazole tautomer isotopologues and vibrationally excited states studied. [ABSTRACT FROM AUTHOR]

The VSEPR model has well-established limitations in its ability to represent accurate molecular and electronic geometries of simple molecules, which can create a significant need for students to relearn structure and bonding concepts in organic chemistry. We present an alternate method for describing molecular geometries and electronic structures to students that is far more consistent with experimental observations than VSEPR. Our alternate method of teaching structures extends VSEPR to account for conjugation to adjacent p-systems and the experimentally observed nature of lone pairs, which is supported by computational chemistry via WebMO's HTML-export feature. If implemented, it allows students to reconcile hybridization, valence bond theory, molecular orbital theory, and resonance structures into a single coherent picture of electronic structure and bonding for organic molecules.

The use of computational molecular modeling to enhance the teaching of chemical concepts is becoming commonplace. Incorporation of this technique into the curriculum, however, typically requires financial investment. This reality poses a problem for institutions where funding and associated resources are scarce and has a potential negative impact on student learning. To address this situation, a free, universally accessible web resource designed to make computational chemistry available to community colleges and similar institutions that lack sufficient resources or expertise to design their own computational modules is presented. The website enables students to view, manipulate, and analyze Gaussian 09 outputs of organic molecules in the front-end user interface WebMO. The associated exercises guide students through key concepts in understanding the structure, bonding, and reactivity of organic molecules. The website and exercises have been used at two different two-year institutions as part of their second-semester organic chemistry courses.

The 1H- and 2H-1,2,3-triazoles are isomeric five-membered ring, aromatic heterocycles that may undergo chemical equilibration by virtue of intramolecular hydrogen migration (tautomerization). Using millimeter-wave spectroscopy in the 130–375 GHz frequency range, we measured the spectroscopic constants for thirteen 1H-1,2,3-triazole and sixteen 2H-1,2,3-triazole isotopologues. Herein, we provide highly accurate and highly precise semi-experimental equilibrium (reSE) structures for the two tautomers based on the spectroscopic constants of each set of isotopologues, together with vibration–rotation interaction and electron-mass distribution corrections calculated using coupled-cluster singles, doubles, and perturbative triples calculations [CCSD(T)/cc-pCVTZ]. The resultant structures are compared with a "best theoretical estimate" (BTE), which has recently been shown to be in exceptional agreement with the semi-experimental equilibrium structures of other aromatic molecules. Bond distances of the 1H tautomer are determined to <0.0008 Å and bond angles to <0.2°. For the 2H tautomer, bond angles are also determined to <0.2°, but bond distances are less precise (2σ ≤ 0.0015). Agreement between BTE and reSE values is discussed. [ABSTRACT FROM AUTHOR]

Hydrazoic acid (HN3) is used as a case study for investigating the accuracy and precision by which a molecular structure—specifically, a semi-experimental equilibrium structure (reSE)—may be determined using current state-of-the-art methodology. The influence of the theoretical corrections for effects of vibration–rotation coupling and electron-mass distribution that are employed in the analysis is explored in detail. The small size of HN3 allowed us to deploy considerable computational resources to probe the basis-set dependence of these corrections using a series of coupled-cluster single, double, perturbative triple [CCSD(T)] calculations with cc-pCVXZ (X = D, T, Q, 5) basis sets. We extrapolated the resulting corrections to the complete basis set (CBS) limit to obtain CCSD(T)/CBS corrections, which were used in a subsequent reSE structure determination. The reSE parameters obtained using the CCSD(T)/cc-pCV5Z corrections are nearly identical to those obtained using the CCSD(T)/CBS corrections, with uncertainties in the bond distances and angles of less than 0.0006 Å and 0.08°, respectively. The previously obtained reSE structure using CCSD(T)/ANO2 agrees with that using CCSD(T)/cc-pCV5Z to within 0.000 08 Å and 0.016° for bond distances and angles, respectively, and with only 25% larger uncertainties, validating the idea that reSE structure determinations can be carried out with significantly smaller basis sets than those needed for similarly accurate, strictly ab initio determinations. Although the purely computational re structural parameters [CCSD(T)/cc-pCV6Z] fall outside of the statistical uncertainties (2σ) of the corresponding reSE structural parameters, the discrepancy is rectified by applying corrections to address the theoretical limitations of the CCSD(T)/cc-pCV6Z geometry with respect to basis set, electron correlation, relativity, and the Born–Oppenheimer approximation, thereby supporting the contention that the semi-experimental approach is both an accurate and vastly more efficient method for structure determinations than is brute-force computation. [ABSTRACT FROM AUTHOR]

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 web-based, 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 pure rotational spectrum of thiazole (c-C3H3NS, Cs) has been studied in the millimeter-wave region from 130 to 375 GHz. Nearly 4800 newly measured rotational transitions for the ground vibrational state of the main isotopologue were combined with previously reported measurements and least-squares fit to a complete sextic Hamiltonian. Transitions for six singly substituted heavy-atom isotopologues (13C, 15N, 33S, 34S) were observed at natural abundance and likewise fit. Several deuterium-enriched samples were prepared, which gave access to the rotational spectra of 16 additional isotopologues, 14 of which had not been previously studied. The rotational spectra of each isotopologue were fit to A- and S-reduced distorted-rotor Hamiltonians in the Ir representation. The experimental values of the ground-state rotational constants (A0, B0, and C0) from each isotopologue were converted to determinable constants (A0″, B0″, and C0″), which were corrected for effects of vibration–rotation interactions and electron-mass distributions using coupled-cluster singles, doubles, and perturbative triples calculations [CCSD(T)/cc-pCVTZ]. The moments of inertia from the resulting constants (Ae, Be, and Ce) of 24 isotopologues were used to determine the precise semi-experimental equilibrium structure (reSE) of thiazole. As a basis for comparison, a purely theoretical equilibrium structure was estimated by an electronic structure calculation [CCSD(T)/cc-pCV5Z] that was subsequently corrected for 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. Some, but not all, of the best theoretical re structural parameters fall within the narrow statistical limits (2σ) of the reSE results. The possible origin of the discrepancies between the best theoretical estimate re and semi-empirical reSE structures is discussed. [ABSTRACT FROM AUTHOR]

The electronic and molecular structure of the acylium cation ([CH[subscript 3]CO][superscript +], 1) receives varied treatment in undergraduate textbooks and online resources. The overall structure of 1 is typically represented as an equal combination of resonance structures containing C-O triple and double bonds, the latter structure occasionally being shown with a bent C-C-O bond angle. This description is inconsistent with available experimental and theoretical data, all of which indicate that 1 is a linear molecule containing a C[triple bar]O bond, and can lead students to a false conception of structure, conjugation, and charge distribution. A set of simple computational exercises is reported that allows students to calculate and rationalize the most accurate resonance representation of 1.

A discovery-based Grignard experiment for a second-year undergraduate organic chemistry course is described. The exclusive Grignard reagent formed by the reaction of 1-bromo-4-fluorobenzene (1) with Mg is 4-fluorophenylmagnesium bromide (2), which is treated with either benzophenone or CO[subscript 2] to produce the corresponding fluorinated alcohol (3) or benzoic acid (4), respectively. The use of a dihalogenated Grignard reagent requires students to discern its reactivity for synthesis. Students predict the chemoselectivity of Grignard reagent formation based on the C-X bond energies of 1 and investigate their predictions by analysis of [superscript 1]H, [superscript 13]C, and [superscript 19]F NMR, EI-MS, and IR data of 3 and 4. Empirical parameters and DFT calculations are used to predict the [superscript 1]H and [superscript 13]C NMR chemical shifts of 4 and the hypothetical brominated analogue.

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. [ABSTRACT FROM AUTHOR]