The impact of surgeon volume on total thyroidectomy outcomes among otolaryngologists.

PURPOSE: To evaluate the impact of surgeon volume on total thyroidectomy complications and outcomes among otolaryngologists. MATERIALS AND METHODS: This state-wide, multi-hospital retrospective review identified patients who underwent total thyroidectomy (TT) (ICD9-06.4) through the Statewide Planning and Research Cooperative System (SPARCS) between 1995 and 2015. Surgeons were categorized into high (>100), medium (10-99), and low (<10) volume groups and differences in complication rates were analyzed. Statistical analysis employed Spearman's rank correlation, Kruskal-Wallis testing, and chi-squared testing. RESULTS: 32,133 TT performed by 1032 otolaryngologists were identified. Overall complication rate in our cohort was 9.83% (CI: 9.48-10.18). The most common complication identified overall was hypocalcemia occurring in 3.85% of cases. Surgeons in the high volume group had a complication rate of 9.6%, compared to 10.0% and 11.6% in the medium and low volume groups. This represents a moderate, but statistically significant difference (rho: -0.4, p < 0.0001; KW p ≤0.0001). When looking at individual complications, temporary tracheostomy rate was higher in the low volume group (5.1%, p = 0.001). Other variables such as advanced age, sex, non-white race, or thyroid malignancy were not predictors of increased complication rates for TT. CONCLUSIONS: Otolaryngologists who perform a high volume of total thyroidectomy were found to have overall less perioperative complications than those with less volume. In particular, the risk of temporary tracheostomy is higher among low volume surgeons. These findings are consistent with previous studies of the effect of thyroidectomy volume on surgical complications.

Local FK506 delivery at the direct nerve repair site improves nerve regeneration.

INTRODUCTION: The objective of this study is to assess the efficacy of local tacrolimus (FK506) delivery to improve outcomes in the setting of nerve transection injury. METHODS: FK506 embedded poly(lactide-co-caprolactone) films capable of extended, localized release of FK506 were developed. FK506 rate of release testing and bioactivity assay was performed. Mouse sciatic nerve transection and direct repair model was used to evaluate the effect extended, local delivery of FK506 had on nerve regeneration outcomes. RESULTS: Linear release of FK506 was observed for 30 days and released FK506 matched control levels of neurite extension in the dorsal root ganglion assay. Groups treated with local FK506 had greater gastrocnemius muscle weight, foot electromyogram, and number of axons distal of the repair site than non-FK506 groups. DISCUSSION: Results of this study indicate that extended, localized delivery of FK506 to nerve injuries can improve nerve regeneration outcomes in a mouse sciatic nerve transection and repair.

Syphilis of the oropharynx: Case series of "The Great Masquerader".

OBJECTIVE: Syphilis is a sexually transmitted infection with various presentations. Although, oropharyngeal manifestations are known to occur, the purpose of this study is to present the first case series in which the lesions were initially mistaken for human-papillomavirus (HPV)-related oropharyngeal squamous cell carcinoma (OPSCC). METHODS: A multi-institutional retrospective review. RESULTS: Six cases of oropharyngeal syphilis were initially thought to be secondary to OPSCC due to presentation. Symptoms were vague and exam findings consisted of either a tonsillar or base of tongue mass, or lymphadenopathy. Biopsies were negative for OPSCC. Further workup diagnosed syphilis, with resolution of symptoms and lesions after antibiotic treatment. CONCLUSIONS: Head and neck manifestations of syphilis have been reported in the literature. However, this is the first series reporting on oropharyngeal syphilis masquerading as HPV-related OPSCC. Ultimately, otolaryngologists must maintain a high suspicion for syphilis in order to ensure prompt diagnosis and treatment.

Fundamental Vibrational Analyses of the HCN Monomer, Dimer and Associated Isotopologues.

In this work we provide high level ab initio treatments of the structures, vibrational frequencies, and electronic energies of the HCN monomer and dimer systems along with several isotopologues. The plethora of information related to this system within the literature is summarized and serves as a basis for comparison with the results of this paper. The geometry of the dimer and monomer are reported at the all electroncoupled-cluster singles, doubles, and perturbative triples level of theory [AE-CCSD(T)] with the correlation consistent quadruple-zeta quality basis sets with extra core functions (cc-pCVQZ) from Dunning. The theoretical geometries and electronic structures are further analyzed through the use of the Natural Bond Orbital (NBO) method and Natural Resonance Theory (NRT). At the AE-CCSD(T)/cc-pCVQZ level of theory, the full cubic with semi-diagonal quartic force field for nine dimer and four monomer isotopologues (the parent isotopologue along with 15 N, 13 C, and D derivatives) were obtained to treat the anharmonicity of the vibrations via second order vibrational perturbation theory (VPT2). Lastly, the enthalpy change associated with the formation of the dimer from two monomer units was determined using the focal point analysis. Computations including coupled-cluster through perturbative quadruples as well as basis sets up to six-zeta quality, including core functions (cc-pCVXZ, X=D,T,Q,5,6) were used to extrapolate to the AE-CCSDT(Q)/CBS energy associated with this hydrogen-bond forming process. After appending anharmonic zero-point vibrational, relativistic, and diagonal Born-Oppenheimer corrections, we report a value of -3.93 kcal mol-1 for the enthalpy of formation. To our knowledge, each set of results (geometries, vibrational frequencies, and energetics) reported in this study represents the highest-level and most reliable theoretical predictions reported for this system.

Using an iterative eigensolver and intertwined rank reduction to compute vibrational spectra of molecules with more than a dozen atoms: Uracil and naphthalene.

We use a direct product basis, basis vectors computed by evaluating matrix-vector products, and rank reduction to calculate vibrational energy levels of uracil and naphthalene, with 12 and 18 atoms, respectively. A matrix representing the Hamiltonian in the direct product basis and vectors with as many components as there are direct product basis functions are neither calculated nor stored. We also introduce an improvement of the Hierarchical Intertwined Reduced-Rank Block Power Method (HI-RRBPM), proposed previously in Thomas and Carrington, Jr. [J. Chem. Phys. 146, 204110 (2017)]. It decreases the memory cost of the HI-RRBPM and enables one to compute vibrational spectra of molecules with over a dozen atoms with a typical desktop computer.

Reinterpreting the infrared spectrum of H + HCN: Methylene amidogen radical and its coproducts.

The methylene amidogen radical (H2CN) plays a role in high-energy material combustion and extraterresterial atmospheres. Recent theoretical work has struggled to match experimental assignments for its CN and antisymmetric CH2 stretching frequencies (ν2 and ν5), which were reported to occur at 1725 and 3103 cm-1. Herein, we compute the vibrational energy levels of this molecule by extrapolating quadruples-level coupled-cluster theory to the complete basis limit and adding corrections for vibrational anharmonicity. This level of theory predicts that ν2 and ν5 should occur at 1646 and 2892 cm-1, at odds with the experimental assignments. To investigate the possibility of defects in our theoretical treatment, we analyze the sensitivity of our approach to each of its contributing approximations. Our analysis suggests that the observed deviation from experiment is too large to be explained as an accumulation of errors, leading us to conclude that these transitions were misassigned. To help resolve this discrepancy, we investigate possible byproducts of the H + HCN reaction, which was the source of H2CN in the original experiment. In particular, we predict vibrational spectra for cis-HCNH, trans-HCNH, and H2CNH using high-level coupled-cluster computations. Based on these results, we reassign the transition at 1725 cm-1 to ν3 of trans-HCNH, yielding excellent agreement. Supporting this identification, we assign a known contaminant peak at 886 cm-1 to ν5 of the same conformer. Our computations suggest that the peak observed at 3103 cm-1, however, does not belong to any of the aforementioned species. To facilitate further investigation, we use structure and bonding arguments to narrow the range of possible candidates. These arguments lead us to tentatively put forth formaldazine [(H2CN)2] as a suggestion for further study, which we support with additional computations.

Ethylperoxy radical: approaching spectroscopic accuracy via coupled-cluster theory.

Interest in peroxy radicals derives from their central role in tropospheric and low-temperature combustion processes; however, their transient nature limits the scope of possible experimental characterization. As a result, theoretical methods (notably, coupled-cluster theory) have become indispensable in the reliable prediction of properties of such ephemeral open-shell systems. Herein, the X[combining tilde] and à state conformers of ethylperoxy radical (C2H5O2) have been structurally optimized at the CCSD(T)/ANO2 level of theory. Relative enthalpies at 0 K [including à ← X[combining tilde] transition origins (T0)] are reported, incorporating CCSD(T) electronic energies extrapolated to the complete basis set limit via the focal point approach. Higher-level computations, employing basis sets as large as cc-pV5Z and post-HF methods up to CCSDT(Q), prove essential in achieving predictions to within 10 cm-1 for experimental T0; we predict 7363 and 7583 cm-1 for the trans and gauche conformers, respectively. Furthermore, predictions of X[combining tilde] state fundamental transitions incorporating CCSD(T)/ANO0 anharmonic contributions are given. For each conformer, all 21 modes were characterized, improving upon the 16 modes reported in the experimental literature, and providing predictions for the 5 remaining modes.

Thioformaldehyde S-Sulfide, Sulfur Analogue of the Criegee Intermediate: Structures, Energetics, and Rovibrational Analysis.

The ephemeral Criegee intermediate, first postulated over 70 years ago, has only recently been isolated in the gas phase. The sulfur analogue of this canonical zwitterion, thioformaldehyde S-sulfide, has eluded similar analysis; however, argon matrix isolation has been achieved ( Angew. Chem., Int. Ed. 2001 , 40 , 393 - 396 ). Here thioformaldehyde S-sulfide and its valence isomer dithiirane are examined with high-level coupled-cluster methods, including the minimum-energy pathway for interconversion. Relative enthalpies calculated from extrapolated energies at the complete basis set limit of the full CCSDTQ method are reported. Isomerization from thioformaldehyde S-sulfide to the lower-lying dithiirane (-7.2 kcal mol-1) is predicted to include a 27.0 kcal mol-1 barrier. Harmonic and anharmonic vibrational frequencies are also predicted using second-order vibrational perturbation theory. These results should aid in future gas-phase identification.

Pleomorphic liposarcoma of the head and neck: Presentation of two cases and literature review.

BACKGROUND: Pleomorphic liposarcoma (PL) is an exceedingly rare tumor of the head and neck. This aggressive liposarcoma variant portends a poorer prognosis compared to more typical sarcomatous tumors. METHODS: Multi-institutional study including two cases of PL, the first case occurring in the post-auricular region and the second in the cheek with later recurrence within the parotid bed. RESULTS: Both patients were treated surgically with wide local excision. One patient required reoperation to obtain negative margins. The other patient underwent a total parotidectomy with neck dissection for recurrent intraparotid nodal disease. Both patients underwent adjuvant radiotherapy. No patient had evidence of recurrent disease with minimum one year follow-up. CONCLUSION: Although the literature is sparse on this disease, wide surgical extirpation with negative margins and adjuvant radiation is recommended. Increased reports of this pathology can be helpful in providing practitioners with experienced-based information that can aid in early detection and treatment. We present the first case series of PL of the head and neck in the literature.

Supraclavicular Flap Reconstruction of Cutaneous Defects Has Lower Complication Rate than Mucosal Defects.

Background The objective of this study was to determine whether there was a difference in complication rate between cutaneous and mucosal defects reconstructed with the supraclavicular artery flap. Methods Retrospective review of postoperative complications in 63 patients from 2008 to 2015 with cutaneous and mucosal head and neck defects following oncologic ablation reconstructed with the supraclavicular flap, with a minimum follow-up duration of 6 months. Of the 63 patients, 38 patients had cutaneous defects, whereas 25 had mucosal defects. Patients were followed up postoperatively to determine the presence of wound infection, partial flap necrosis, complete flap necrosis, and fistula formation. Complications in both defect groups as well as a statistical comparison of total complications were analyzed. Results Patients with cutaneous defects reconstructed with the supraclavicular flap had significantly lower postoperative complications than those with mucosal defects (p = 0.002). Flap necrosis, both partial and complete, was also lower in this same group (p = 0.0052). Conclusion The supraclavicular artery flap may be a more suitable option for patients with cutaneous defects, given the reliability and lower propensity for postoperative complications Level of Evidence The level of evidence is 4.

Mn-NHC Electrocatalysts: Increasing π Acidity Lowers the Reduction Potential and Increases the Turnover Frequency for CO2 Reduction.

A new manganese(I) N-heterocyclic carbene electrocatalyst containing a benzimidazole-pyrimidine-based ligand is reported for the two-electron conversion of CO2. The increased π acidity of pyrimidine shifts the two-electron reduction to -1.77 V vs Fc/Fc+, 70 mV more positive than that for MnBr(2,2'-bipyridine)(CO)3; increased catalytic current enhancement is also observed (5.2× vs 2.1×). Theoretical analyses suggest that this heightened activity may follow from the preference for a reduction-first dehydroxylation mechanism.

Photocatalytic Reduction of Carbon Dioxide to CO and HCO2H Using fac-Mn(CN)(bpy)(CO)3.

Studies are reported regarding the use of Mn(CN)(bpy)(CO)3 (1) as a catalyst for CO2 reduction employing [Ru(dmb)3](2+) as a photosensitizer in mixtures of dry N,N-dimethylformamide-triethanolamine (N,N-DMF-TEOA) or acetonitrile-TEOA (MeCN-TEOA) with 1-benzyl-1,4-dihydronicotinamide as a sacrificial reductant. Irradiation with 470 nm light for up to 15 h yields both CO and HCO2H with maximum turnover numbers (TONs) as high as 21 and 127, respectively, with product preference dependent on the solvent. Further data suggests that upon single electron reduction this catalyst avoids the formation of a Mn-Mn dimer and instead undergoes a disproportionation reaction, which requires 2 equiv of [Mn(CN)(bpy)(CO)3](•-) to generate 1 equiv each of the active catalyst [Mn(bpy)(CO)3](-) and the starting compound 1. Additional characterization by cyclic voltammetry (CV) and infrared spectroelectrochemistry (IR-SEC) indicates that the stability of the singly reduced [Mn(CN)(bpy)(CO)3](•-) differs slightly in the N,N-DMF-TEOA solvent system compared to the MeCN-TEOA system. This contributes to the observed selectivities for HCO2H vs CO production.

Re(I) NHC Complexes for Electrocatalytic Conversion of CO2.

The modular construction of ligands around an N-heterocyclic carbene building block represents a flexible synthetic strategy for tuning the electronic properties of metal complexes. Herein, methylbenzimidazolium-pyridine and methylbenzimidazolium-pyrimidine proligands are constructed in high yield using recently established transition-metal-free techniques. Subsequent chelation to ReCl(CO)5 furnishes ReCl(N-methyl-N'-2-pyridylbenzimidazol-2-ylidine)(CO)3 and ReCl(N-methyl-N'-2-pyrimidylbenzimidazol-2-ylidine)(CO)3. These Re(I) NHC complexes are shown to be capable of mediating the two-electron conversion of CO2 following one-electron reduction; the Faradaic efficiency for CO formation is observed to be >60% with minor H2 and HCO2H production. Data from cyclic voltammetry is presented and compared to well-studied ReCl(2,2'-bipyridine)(CO)3 and MnBr(2,2'-bipyridine)(CO)3 systems. Results from density functional theory computations, infrared spectroelectrochemistry, and chemical reductions are also discussed.

Characterizing a nonclassical carbene with coupled cluster methods: cyclobutylidene.

Carbenes represent a special class of reactive compounds that possess a lone pair of electrons on a carbon atom. Among the myriad examples of carbenes in the literature, cyclobutylidene stands out as a unique nonclassical compound that includes transannular interaction between opposing C1 and C3 carbon atoms within a four-membered ring. On its lowest potential energy surface (X[combining tilde](1)A'), cyclobutylidene quickly rearranges, following three reaction paths: (i) 1,2-H migration; (ii) 1,2-C migration; and, (iii) 1,3-H migration. Herein, this reactivity is examined with high-level coupled-cluster methods [up to CCSDT(Q)]. At this level of theory, combined with extrapolation techniques to obtain energies at the complete basis set (CBS) limit, the long-standing disparity between theoretical and experimental results is resolved. Specifically, cyclobutylidene is predicted to prefer 1,2-C migration rather than 1,2-H migration. Rate constants for the three reaction paths are obtained from canonical variational transition state theory (CVT) and yield reasonable agreement with existing experimental results. Further characterization of cyclobutylidene is also reported: the singlet-triplet gap (ΔES-T) is found to be -9.3 kcal mol(-1) at the CCSDT(Q)/CBS level of theory, and anharmonic vibrational frequencies are determined with second-order vibrational perturbation theory (VPT2).

σ Bond activation through tunneling: formation of the boron hydride cations BHn(+) (n = 2, 4, 6).

The network of H2 additions to B(+) and subsequent insertion reactions serve as a tractable model for hydrogen storage in elementary boron-containing compounds. Here, they are investigated using state-of-the-art ab initio methods (up to CCSDTQ and cc-pCV6Z basis sets). The binding energies of H2 to HBH(+) (14.9 kcal mol(-1)) and HBH(H2)(+) (18.1 kcal mol(-1)) are determined to be much higher than those for B(H2)(+) (3.8 kcal mol(-1)), B(H2)2(+) (3.0 kcal mol(-1)), and B(H2)3(+) (2.5 kcal mol(-1)) at the CCSDTQ/CBS level of theory. These predictions are in agreement with the experiments of Kemper, Bushnell, Weis, and Bowers (J. Am. Chem. Soc., 1998, 120, 7577). Molecular orbital analyses show that the enhanced binding in HBH(H2)m(+) complexes originates from the strong interaction between the 1σu HOMO of HBH(+) and the 1σu LUMO of H2. For the insertion reactions B(H2)n(+) → HBH(H2)n-1(+), activation barriers are determined to be 58.3 kcal mol(-1) [Mk-MRCCSD(T)/CBS], 12.2 kcal mol(-1) (CCSDTQ/CBS) and 4.6 kcal mol(-1) (CCSDTQ/CBS) for n = 1, 2, and 3, respectively. After using theoretical results to remove tunneling effects from the experimental rate constants, new Arrhenius fits yield activation barriers of 4.6(3) kcal mol(-1) and 3.8(1) kcal mol(-1) for the BH6(+) and BD6(+) insertion reactions, respectively, which are in near perfect agreement with converged theoretical values (4.6 kcal mol(-1) and 3.9 kcal mol(-1)). These findings demonstrate that earlier Arrhenius fits considerably underestimate these barriers, and that quantum tunneling dominates the σ bond activation mechanism witnessed in previous experiments involving BH6(+).

Energetics and transition-state dynamics of the F + HOCH3 → HF + OCH3 reaction.

The F + HOCH3 → HF + OCH3 reaction is a system with 15 internal degrees of freedom that can provide a benchmark for the development of theory for increasingly complex chemical reactions. The dynamics of this reaction were studied by photoelectron-photofragment coincidence (PPC) spectroscopy carried out on the F-(HOCH3) anion, aided by a computational study of both the anion and neutral potential energy surfaces, with energies extrapolated to the CCSDT(Q)/CBS level of theory. Photodetachment at 4.80 eV accesses both the reactant and product channels for this reaction. In the product channel (HF + OCH3 + e-) of the neutral potential energy surface, vibrationally excited HF products in addition to the stable product-channel hydrogen-bonded complex (FH-OCH3) are observed in the PPC and photoelectron spectra. In addition, experimental evidence is observed for the reactant-channel van der Waals complex (F-HOCH3), in good agreement with the theoretical predictions. The relative stability of these long-lived complexes was probed by reducing the ion beam energy, increasing the product time-of-flight, indicating lifetimes on the microsecond timescale for the reactant- and product-channel complexes as well as providing evidence for long-lived vibrational Feshbach resonances associated with the HF(v > 0) + OCH3 product states. This system will provide a model for extending full-dimensionality quantum dynamics to larger numbers of degrees of freedom.

Sub-Doppler infrared spectroscopy and formation dynamics of triacetylene in a slit supersonic expansion.

Infrared spectroscopy and formation dynamics of triacetylene are investigated in a slit jet supersonic discharge and probed with sub-Doppler resolution (≈60 MHz) on the fundamental antisymmetric CH stretch mode (ν5). The triacetylene is generated in the throat of the discharge by sequential attack of ethynyl radical with acetyelene and diacetylene: (i) HCCH → HCC + H, (ii) HCC + HCCH → HCCCCH + H, (iii) HCC + HCCCCH → HCCCCCCH + H, cooled rapidly in the slit expansion to 15 K, and probed by near shot-noise-limited absorption sensitivity with a tunable difference-frequency infrared laser. The combination of jet cooled temperatures (Trot = 15 K) and low spectral congestion permits (i) analysis of rotationally avoided crossings in the ν5 band ascribed to Coriolis interactions, as well as (ii) first detection of ν5 Π-Π hot band progressions built on the ν12 sym CC bend and definitively assigned via state-of-the-art ab initio vibration-rotation interaction parameters (αi), which make for interesting comparison with recent spectroscopic studies of Doney et al. [J. Mol. Spectrosc. 316, 54 (2015)]. The combined data provide direct evidence for significantly non-equilibrium populations in the CC bending manifold, dynamically consistent with a strongly bent radical intermediate and transition states for forming triacetylene product. The presence of intense triacetylene signals under cold, low density slit jet conditions provides support for (i) barrierless addition of HCC with HCCCCH and (ii) a high quantum yield for HCCCCCCH formation. Complete basis set calculations for energetics [CCSD(T)-f12/VnZ-f12, n = 2,3] and frequencies [CCSD(T)-f12/VdZ-f12] are presented for both radical intermediate and transition state species, predicting collision stabilization in the slit jet expansion to be competitive with unimolecular decomposition with increasing polyyne chain length.

Infrared laser spectroscopy of the n-propyl and i-propyl radicals: Stretch-bend Fermi coupling in the alkyl CH stretch region.

The n-propyl and i-propyl radicals were generated in the gas phase via pyrolysis of n-butyl nitrite [CH3(CH2)3ONO] and i-butyl nitrite [(CH3)2CHCH2ONO], respectively. Nascent radicals were promptly solvated by a beam of He nanodroplets, and the infrared spectra of the radicals were recorded in the CH stretching region. Several previously unreported bands are observed between 2800 and 3150 cm-1. The CH stretching modes observed above 3000 cm-1 are in excellent agreement with CCSD(T) anharmonic frequencies computed using second-order vibrational perturbation theory. However, between 2800 and 3000 cm-1, the spectra of n- and i-propyl radicals become congested and difficult to assign due to the presence of multiple anharmonic resonance polyads. To model the spectrally congested region, Fermi and Darling-Dennison resonances are treated explicitly using "dressed" Hamiltonians and CCSD(T) quartic force fields in the normal mode representation, and the agreement with experiment is less than satisfactory. Computations employing local mode effective Hamiltonians reveal the origin of the spectral congestion to be strong coupling between the high frequency CH stretching modes and the lower frequency CHn bending/scissoring motions. The most significant coupling is between stretches and bends localized on the same CH2/CH3 group. Spectral simulations using the local mode approach are in excellent agreement with experiment.

The effect of high flow nasal cannula oxygen therapy on middle ear pressure.

OBJECTIVES: To investigate the effect of high flow nasal cannula oxygen therapy (HFOT) on middle ear pressure. MATERIALS/METHODS: Ten patients (eight males and two females) with oxygen desaturations requiring HFOT were recruited with 19 ears available for our study. The study group was aged 29-90years (mean 65.3±16.5). All patients underwent a review of medical history, questioned about subjective hearing loss and underwent a standard otologic exam, with middle ear pressures measured with a GSI TympStar tympanometer. RESULTS: The middle ear peak pressures in our study group ranged from 25 to -200daPa (mean -13.7±56.3daPa). Volume of HFOT was delivered at 20-40L (mean 30.5±9L) and fraction of inspired oxygen required was 30-70% (mean 58±13%). There was a positive correlation between liters of oxygen delivery and middle ear pressure with a Pearson coefficient (R) of 0.436, although lacking statistical significance (p=0.06). CONCLUSION: Previous studies have shown that HFOT delivered in the range of 35-40L/min produces pharyngeal pressures at or above 5cm H2O. Since pharyngeal pressures of 5cm H2O produced via CPAP have shown to produce middle ear pressures above 40daPa, we expected HFOT to result in similar middle ear pressures of 35-40L/min. However, although our results show an increase in middle ear pressures with flow volume, HFOT did not produce significant increases in middle ear pressures. This may make HFOT an appropriate option of oxygen delivery to patients who require otologic procedures.

Dehydropericyclic Reactions: Symmetry-Controlled Routes to Strained Reactive Intermediates.

The conceptual dehydrogenation of pericyclic reactions yields dehydropericyclic processes, which usually lead to strained or reactive intermediates. This is a simple scheme for inventing new chemical reactions. Computational results on two novel dehydropericyclic reactions are presented here. Conjugated enynes undergo a singlet-state photoisomerization that transposes the methylene carbon. We previously suggested excited-state closure to 1,2-cyclobutadiene followed by thermal ring opening. CCSD(T)//DFT computations show two minima of similar energy corresponding to 1,2-cyclobutadiene, one chiral and closed shell and the second a planar diradical. The chiral structure has a low barrier to ring opening and may best explain results on enyne photoisomerization. The first examples of 1,3-diyne + yne cycloadditions to give o-benzynes were reported in 1997. Computations on intramolecular versions of this tridehydro (-3H2) Diels-Alder reaction support a concerted mechanism for the parent triyne (1,3,8-nonatriyne); however, a slight electronic advantage in the concerted path may be outweighed by the difference in entropy of activation for sequential vs simultaneous formation of two new ring bonds.