Rotational Spectrum of the Phenoxy Radical.

We report the hyperfine-resolved rotational spectrum of the gas-phase phenoxy radical in the 8-25 GHz frequency range using cavity Fourier transform microwave spectroscopy. A complete assignment of its complex but well-resolved fine and hyperfine splittings yielded a precisely determined set of rotational constants, spin-rotation parameters, and nuclear hyperfine coupling constants. These results are interpreted with support from high-level quantum chemical calculations to gain detailed insight into the distribution of the unpaired π electron in this prototypical resonance-stabilized radical. The accurate laboratory rest frequencies enable studies of the chemistry of phenoxy in both the laboratory and space. The prospects of extending the present experimental and theoretical techniques to investigate the rotational spectra of isotopic variants and structural isomers of phenoxy and other important gas-phase radical intermediates that are yet undetected at radio wavelengths are discussed.

Direct Probes of π-Delocalization in Prototypical Resonance-Stabilized Radicals: Hyperfine-Resolved Microwave Spectroscopy of Isotopic Propargyl and Cyanomethyl.

Delocalization of the unpaired electron in π-conjugated radicals has profound implications for their chemistry, but direct and quantitative characterization of this electronic structure in isolated molecules remains challenging. We apply hyperfine-resolved microwave rotational spectroscopy to rigorously probe π-delocalization in propargyl, CH2CCH, a prototypical resonance-stabilized radical and key reactive intermediate. Using the spectroscopic constants derived from the high-resolution cavity Fourier transform microwave measurements of an exhaustive set of 13C- and 2H-substituted isotopologues, together with high-level ab initio calculations of zero-point vibrational effects, we derive its precise semiexperimental equilibrium geometry and quantitatively characterize the spatial distribution of its unpaired electron. Our results highlight the importance of considering both spin-polarization and orbital-following contributions when interpreting the isotropic hyperfine coupling constants of π radicals. These physical insights are strengthened by a parallel analysis of the isoelectronic species cyanomethyl, CH2CN, using new 13C measurements also reported in this work. A detailed comparison of the structure and electronic properties of propargyl, cyanomethyl, and other closely related species allows us to correlate trends in their chemical bonding and electronic structure with critical changes in their reactivity and thermochemistry.

Ergodicity breaking in rapidly rotating C60 fullerenes.

Ergodicity, the central tenet of statistical mechanics, requires an isolated system to explore all available phase space constrained by energy and symmetry. Mechanisms for violating ergodicity are of interest for probing nonequilibrium matter and protecting quantum coherence in complex systems. Polyatomic molecules have long served as a platform for probing ergodicity breaking in vibrational energy transport. Here, we report the observation of rotational ergodicity breaking in an unprecedentedly large molecule, 12C60, determined from its icosahedral rovibrational fine structure. The ergodicity breaking occurs well below the vibrational ergodicity threshold and exhibits multiple transitions between ergodic and nonergodic regimes with increasing angular momentum. These peculiar dynamics result from the molecule's distinctive combination of symmetry, size, and rigidity, highlighting its relevance to emergent phenomena in mesoscopic quantum systems.

Structural and electronic trends of optical cycling centers in polyatomic molecules revealed by microwave spectroscopy of MgCCH, CaCCH, and SrCCH.

The unique optical cycling efficiency of alkaline earth metal-ligand molecules has enabled significant advances in polyatomic laser cooling and trapping. Rotational spectroscopy is an ideal tool for probing the molecular properties that underpin optical cycling, thereby elucidating the design principles for expanding the chemical diversity and scope of these platforms for quantum science. We present a comprehensive study of the structure and electronic properties in alkaline earth metal acetylides with high-resolution microwave spectra of 17 isotopologues of MgCCH, CaCCH, and SrCCH in their 2Σ+ ground electronic states. The precise semiexperimental equilibrium geometry of each species has been derived by correcting the measured rotational constants for electronic and zero-point vibrational contributions calculated with high-level quantum chemistry methods. The well-resolved hyperfine structure associated with the 1,2H, 13C, and metal nuclear spins provides further information on the distribution and hybridization of the metal-centered, optically active unpaired electron. Together, these measurements allow us to correlate trends in chemical bonding and structure with the electronic properties that promote efficient optical cycling essential to next-generation experiments in precision measurement and quantum control of complex polyatomic molecules.

Hyperfine-Resolved Rotational Spectroscopy of Phenyl Radical.

We present an analysis of the hyperfine-resolved rotational spectrum of gas-phase phenyl radical, c-C6H5, between 9 and 35 GHz. The isotropic and anisotropic hyperfine parameters of all five protons and the electronic spin-rotation fine structure parameters are accurately determined from this study, which allow detailed insight into the distribution and interactions of the unpaired electron in this prototypical σ-radical. The implications for laboratory and astronomical investigations of phenyl that are reliant on a precise centimeter-wave catalog are discussed, as are the prospects for detecting and assigning the hyperfine-resolved rotational spectra of other large, weakly polar hydrocarbon chain and ring radicals.

Franck-Condon spectra of unbound and imaginary-frequency vibrations via correlation functions: A branch-cut free, numerically stable derivation.

Molecular electronic spectra can be represented in the time domain as auto-correlation functions of the initial vibrational wavepacket. We present a derivation of the harmonic vibrational auto-correlation function that is valid for both real and imaginary harmonic frequencies. The derivation rests on Lie algebra techniques that map otherwise complicated exponential operator arithmetic to simpler matrix formulas. The expressions for the zero- and finite-temperature harmonic auto-correlation functions have been carefully structured both to be free of branch-cut discontinuities and to remain numerically stable with finite-precision arithmetic. Simple extensions correct the harmonic Franck-Condon approximation for the lowest-order anharmonic and Herzberg-Teller effects. Quantitative simulations are shown for several examples, including the electronic absorption spectra of F2, HOCl, CH2NH, and NO2.

Ring-Opening Dynamics of the Cyclopropyl Radical and Cation: the Transition State Nature of the Cyclopropyl Cation.

We provide compelling experimental and theoretical evidence for the transition state nature of the cyclopropyl cation. Synchrotron photoionization spectroscopy employing coincidence techniques together with a novel simulation based on high-accuracy ab initio calculations reveal that the cation is unstable via its allowed disrotatory ring-opening path. The ring strains of the cation and the radical are similar, but both ring opening paths for the radical are forbidden when the full electronic symmetries are considered. These findings are discussed in light of the early predictions by Longuet-Higgins alongside Woodward and Hoffman; we also propose a simple phase space explanation for the appearance of the cyclopropyl photoionization spectrum. The results of this work allow the refinement of the cyclopropane C-H bond dissociation energy, in addition to the cyclopropyl radical and cation cyclization energies, via the Active Thermochemical Tables approach.

A Search for Heterocycles in GOTHAM Observations of TMC-1.

We have conducted an extensive search for nitrogen-, oxygen-, and sulfur-bearing heterocycles toward Taurus Molecular Cloud 1 (TMC-1) using the deep, broadband centimeter-wavelength spectral line survey of the region from the GOTHAM large project on the Green Bank Telescope. Despite their ubiquity in terrestrial chemistry, and the confirmed presence of a number of cyclic and polycyclic hydrocarbon species in the source, we find no evidence for the presence of any heterocyclic species. Here, we report the derived upper limits on the column densities of these molecules obtained by Markov Chain Monte Carlo (MCMC) analysis and compare this approach to traditional single-line upper limit measurements. We further hypothesize why these molecules are absent in our data, how they might form in interstellar space, and the nature of observations that would be needed to secure their detection.

Elaborated thermochemical treatment of HF, CO, N2, and H2O: Insight into HEAT and its extensions.

Empirical, highly accurate non-relativistic electronic total atomization energies (eTAEs) are established by combining experimental or computationally converged treatments of the nuclear motion and relativistic contributions with the total atomization energies of HF, CO, N2, and H2O obtained from the Active Thermochemical Tables. These eTAEs, which have estimated (2σ) uncertainties of less than 10 cm-1 (0.12 kJ mol-1), form the basis for an analysis of high-level ab initio quantum chemical calculations that aim at reproducing these eTAEs for the title molecules. The results are then employed to analyze the performance of the high-accuracy extrapolated ab initio thermochemistry, or High-Accuracy Extrapolated Ab Initio Thermochemistry (HEAT), family of theoretical methods. The method known as HEAT-345(Q), in particular, is found to benefit from fortuitous error cancellation between its treatment of the zero-point energy, extrapolation errors in the Hartree-Fock and coupled cluster contributions, neglect of post-(T) core-correlation, and the basis-set error involved in higher-level correlation corrections. In addition to shedding light on a longstanding curiosity of the HEAT protocol-where the cheapest HEAT-345(Q) performs comparably to the theoretically more complete HEAT-456QP procedure-this study lays the foundation for extended HEAT variants that offer substantial improvements in accuracy relative to the established approaches.

Ultrasensitive multispecies spectroscopic breath analysis for real-time health monitoring and diagnostics.

Breath analysis enables rapid, noninvasive diagnostics, as well as long-term monitoring of human health, through the identification and quantification of exhaled biomarkers. Here, we demonstrate the remarkable capabilities of mid-infrared (mid-IR) cavity-enhanced direct-frequency comb spectroscopy (CE-DFCS) applied to breath analysis. We simultaneously detect and monitor as a function of time four breath biomarkers-[Formula: see text]OH, [Formula: see text], [Formula: see text]O, and HDO-as well as illustrate the feasibility of detecting at least six more ([Formula: see text]CO, [Formula: see text], OCS, [Formula: see text], [Formula: see text], and [Formula: see text]) without modifications to the experimental apparatus. We achieve ultrahigh detection sensitivity at the parts-per-trillion level. This is made possible by the combination of the broadband spectral coverage of a frequency comb, the high spectral resolution afforded by the individual comb teeth, and the sensitivity enhancement resulting from a high-finesse cavity. Exploiting recent advances in frequency comb, optical coating, and photodetector technologies, we can access a large variety of biomarkers with strong carbon-hydrogen-bond spectral signatures in the mid-IR.

Exhaustive Product Analysis of Three Benzene Discharges by Microwave Spectroscopy.

Using chirped and cavity microwave spectroscopies, automated double resonance, new high-speed fitting and deep learning algorithms, and large databases of computed structures, the discharge products of benzene alone, or in combination with molecular oxygen or nitrogen, have been exhaustively characterized between 6.5 and 26 GHz. In total, more than 3300 spectral features were observed; 89% of these, accounting for 97% of the total intensity, have now been assigned to 152 distinct chemical species and 60 of their variants (i.e., isotopic species and vibrationally excited states). Roughly 50 of the products are entirely new or poorly characterized at high resolution, including many heavier by mass than the precursor benzene. These findings provide direct evidence for a rich architecture of two- and three-dimensional carbon and indicate that benzene growth, particularly the formation of ring-chain molecules, occurs facilely under our experimental conditions. The present analysis also illustrates the utility of microwave spectroscopy as a precision tool for complex mixture analysis, irrespective of whether the rotational spectrum of a product species is known a priori or not. From this large quantity of data, for example, it is possible to determine with confidence the relative abundances of different product masses, but more importantly the relative abundances of different isomers with the same mass. The complementary nature of this type of analysis to traditional mass spectrometry is discussed.

Synchrotron-Based High Resolution Far-Infrared Spectroscopy of trans-Butadiene.

The high resolution far-infrared spectrum of trans-butadiene has been reinvestigated by Fourier-transform spectroscopy at two synchrotron radiation facilities, SOLEIL and the Canadian Light Source, at temperatures ranging from 50 to 340 K. Beyond the well-studied bands, two new fundamental bands lying below 1100 cm-1, ν10 and ν24, have been assigned using a combination of cross-correlation (ASAP software) and Loomis-Wood type (LWWa software) diagrams. While the ν24 analysis was rather straightforward, ν10 exhibits obvious signs of a strong perturbation, presumably owing to interaction with the dark ν9 + ν12 state. Effective rotational constants have been derived for both the v10 = 1 and v24 = 1 states. Since only one weak, infrared active fundamental band (ν23) of trans-butadiene remains to be observed at high resolution in the far-infrared, searches for the elusive gauche conformer can now be undertaken with considerably greater confidence in the dense ro-vibrational spectrum of the trans form.

Photodissociation transition states characterized by chirped pulse millimeter wave spectroscopy.

The 193-nm photolysis of CH2CHCN illustrates the capability of chirped-pulse Fourier transform millimeter-wave spectroscopy to characterize transition states. We investigate the HCN, HNC photofragments in highly excited vibrational states using both frequency and intensity information. Measured relative intensities of J = 1-0 rotational transition lines yield vibrational-level population distributions (VPD). These VPDs encode the properties of the parent molecule transition state at which the fragment molecule was born. A Poisson distribution formalism, based on the generalized Franck-Condon principle, is proposed as a framework for extracting information about the transition-state structure from the observed VPD. We employ the isotopologue CH2CDCN to disentangle the unimolecular 3-center DCN elimination mechanism from other pathways to HCN. Our experimental results reveal a previously unknown transition state that we tentatively associate with the HCN eliminated via a secondary, bimolecular reaction.

Rotational Characterization of the Elusive gauche-Isoprene.

Isoprene (2-methyl-1,3-butadiene) is highly abundant in the atmosphere, second only to methane in hydrocarbon emissions. In contrast to the most stable trans rotamer, structural characterization of gauche-isoprene has proven challenging: it is weakly polar, present at the level of only a few percent at room temperature, and structurally complex due to both torsional and methyl tunneling motions. gauche-Isoprene has been observed by two distinct but complementary experimental approaches: chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy coupled with cryogenic buffer gas cooling, and cavity-enhanced FTMW spectroscopy with a pulsed discharge source. Thermal enhancement of the gauche population (from 1.7% to 10.3%) was observed in the cryogenic buffer gas cell when the sample was preheated from 300 to 450 K, demonstrating that high-energy rotamers can be efficiently isolated under our experimental conditions. Rotational parameters for the inversion states (0+/0-) have been determined for the first time, aided by calculations at increasing levels of theoretical sophistication. From this combined analysis, the inversion splitting Δ E and the Fbc Coriolis coupling constant between the two inversion states have been derived.

Rovibrational quantum state resolution of the C60 fullerene.

The unique physical properties of buckminsterfullerene, C60, have attracted intense research activity since its original discovery. Total quantum state-resolved spectroscopy of isolated C60 molecules has been of particularly long-standing interest. Such observations have, to date, been unsuccessful owing to the difficulty in preparing cold, gas-phase C60 in sufficiently high densities. Here we report high-resolution infrared absorption spectroscopy of C60 in the 8.5-micron spectral region (1180 to 1190 wave number). A combination of cryogenic buffer-gas cooling and cavity-enhanced direct frequency comb spectroscopy has enabled the observation of quantum state-resolved rovibrational transitions. Characteristic nuclear spin statistical intensity patterns confirm the indistinguishability of the 60 carbon-12 atoms, while rovibrational fine structure encodes further details of the molecule's rare icosahedral symmetry.

The Hunt for Elusive Molecules: Insights from Joint Theoretical and Experimental Investigations.

Rotational spectroscopy is an invaluable tool to unambiguously determine the molecular structure of a species, and sometimes even to establish its very existence. This article illustrates how experimental and theoretical state-of-the-art tools can be used in tandem to investigate the rotational structure of molecules, with particular emphasis on those that have long remained elusive. The examples of three emblematic species-gauche-butadiene, disilicon carbide, and germanium dicarbide-highlight the close, mutually beneficial interaction between high-level theoretical calculations and sensitive microwave measurements. Prospects to detect other elusive molecules of chemical and astronomical interest are discussed.

Fourth-order vibrational perturbation theory with the Watson Hamiltonian: Report of working equations and preliminary results.

A derivation of fourth-order vibrational perturbation theory (VPT4) based on the Watson Hamiltonian in dimensionless rectilinear normal coordinates is presented. Terms that are linear and cubic in the (nk + 12 ), with nk being the zeroth-order harmonic oscillator quantum numbers, appear in fourth order and extend the much simpler second-order vibrational perturbation theory model. The rather involved expressions for the fourth-order terms are derived with Rayleigh-Schrödinger perturbation theory, the process of verifying their correctness is described, and a computer code to generate the VPT4 constants from the potential energy surface derivatives is provided. The paper concludes with numerical examples featuring the H2O, Si2C, and cyclic-C3H2 molecules.

Direct measurements of DOCO isomers in the kinetics of OD + CO.

Quantitative and mechanistically detailed kinetics of the reaction of hydroxyl radical (OH) with carbon monoxide (CO) have been a longstanding goal of contemporary chemical kinetics. This fundamental prototype reaction plays an important role in atmospheric and combustion chemistry, motivating studies for accurate determination of the reaction rate coefficient and its pressure and temperature dependence at thermal reaction conditions. This intricate dependence can be traced directly to details of the underlying dynamics (formation, isomerization, and dissociation) involving the reactive intermediates cis- and trans-HOCO, which can only be observed transiently. Using time-resolved frequency comb spectroscopy, comprehensive mechanistic elucidation of the kinetics of the isotopic analog deuteroxyl radical (OD) with CO has been realized. By monitoring the concentrations of reactants, intermediates, and products in real time, the branching and isomerization kinetics and absolute yields of all species in the OD + CO reaction are quantified as a function of pressure and collision partner.

The Molecular Structure of gauche-1,3-Butadiene: Experimental Establishment of Non-planarity.

The planarity of the second stable conformer of 1,3-butadiene, the archetypal diene for the Diels-Alder reaction in which a planar conjugated diene and a dienophile combine to form a ring, is not established. The most recent high level calculations predicted the species to adopt a twisted, gauche structure owing to steric interactions between the inner terminal hydrogens rather than a planar, cis structure favored by the conjugation of the double bonds. The structure cis-1,3-butadiene is unambiguously confirmed experimentally to indeed be gauche with a substantial dihedral angle of 34°, in excellent agreement with theory. Observation of two tunneling components indicates that the molecule undergoes facile interconversion between two equivalent enantiomeric forms. Comparison of experimentally determined structures for gauche- and trans-butadiene provides an opportunity to examine the effects of conjugation and steric interactions.

Active Thermochemical Tables: The Adiabatic Ionization Energy of Hydrogen Peroxide.

The adiabatic ionization energy of hydrogen peroxide (HOOH) is investigated, both by means of theoretical calculations and theoretically assisted reanalysis of previous experimental data. Values obtained by three different approaches: 10.638 ± 0.012 eV (purely theoretical determination), 10.649 ± 0.005 eV (reanalysis of photoelectron spectrum), and 10.645 ± 0.010 eV (reanalysis of photoionization spectrum) are in excellent mutual agreement. Further refinement of the latter two values to account for asymmetry of the rotational profile of the photoionization origin band leads to a reduction of 0.007 ± 0.006 eV, which tends to bring them into even closer alignment with the purely theoretical value. Detailed analysis of this fundamental quantity by the Active Thermochemical Tables approach, using the present results and extant literature, gives a final estimate of 10.641 ± 0.006 eV.