Butadiene and Heterodienes Revisited

Surprising features in a recently published high-level calculation of the rotational profile of butadiene led us to compare butadiene with a set of 17 heterodienes. The rotational profiles for this large group of compounds varied widely, thereby possessing a high information content. These data were subjected to a Fourier analysis yielding 1- through 6-fold terms: the one-fold terms represent the change in steric energy on going from 180° to 0°, while the changes in the 2-fold terms correspond to the expected change in π-delocalization energy with structure; the 3-fold terms were significant and found to be linearly correlated to the average of the atomic charges of the atoms at the central single bond of the <i>cis</i>-forms, but their origins are still not clear; we propose a novel 1,4 π-interactions that may account for this phenomenon. The 4-fold terms were at times comparable in magnitude to the 3-fold terms but overall appeared to mainly modify the 3-fold terms slightly without introducing any qualitatively new features. The 5- and 6-fold terms were negligible

Laser-Induced Fluorescence Study of the S₁ State of Doubly-Substituted ¹³C Acetylene and Harmonic Force Field Determination

In the first half of this study, rotational and vibrational constants of six Franck–Condon bright vibrational levels of S₁ doubly-substituted ¹³C acetylene are determined from laser-induced fluorescence spectra and an updated geometry of the trans conformer of S₁ acetylene is obtained. In the second half, we determine the quadratic force field of S₁ acetylene on the basis of the harmonic frequencies of four isotopologues of acetylene. The effects of both diagonal and off-diagonal <i>x</i>_<i>ij</i> anharmonicities are removed from the input harmonic frequencies. Results from both experimental and theoretical studies of various isotopologues of acetylene (including those from the first half of this paper) are used to obtain a set of force constants that agrees well with ab initio calculations. Our set of force constants for S₁ acetylene is an improvement over previous work by Tobiason et al., which did not include off-diagonal anharmonicities

Isomerization and Fragmentation of Cyclohexanone in a Heated Micro-Reactor

The thermal decomposition of cyclohexanone (C₆H₁₀O) has been studied in a set of flash-pyrolysis microreactors. Decomposition of the ketone was observed when dilute samples of C₆H₁₀O were heated to 1200 K in a continuous flow microreactor. Pyrolysis products were detected and identified by tunable VUV photoionization mass spectroscopy and by photoionization appearance thresholds. Complementary product identification was provided by matrix infrared absorption spectroscopy. Pyrolysis pressures were roughly 100 Torr, and contact times with the microreactors were roughly 100 μs. Thermal cracking of cyclohexanone appeared to result from a variety of competing pathways, all of which open roughly simultaneously. Isomerization of cyclohexanone to the enol, cyclohexen-1-ol (C₆H₉OH), is followed by retro-Diels–Alder cleavage to CH₂CH₂ and CH₂C­(OH)–CHCH₂. Further isomerization of CH₂C­(OH)–CHCH₂ to methyl vinyl ketone (CH₃CO–CHCH₂, MVK) was also observed. Photoionization spectra identified both enols, C₆H₉OH and CH₂C­(OH)–CHCH₂, and the ionization threshold of C₆H₉OH was measured to be 8.2 <i> ± </i> 0.1 eV. Coupled cluster electronic structure calculations were used to establish the energetics of MVK. The heats of formation of MVK and its enol were calculated to be Δ_f<i>H</i>₂₉₈(<i>cis</i>-CH₃CO–CHCH₂) = −26.1 ± 0.5 kcal mol^–1 and Δ_f<i>H</i>₂₉₈(<i>s-cis</i>-1-CH₂C­(OH)–CHCH₂) = −13.7 ± 0.5 kcal mol^–1. The reaction enthalpy Δ_rxn<i>H</i>₂₉₈(C₆H₁₀O → CH₂CH₂ + <i>s-cis</i>-1-CH₂C­(OH)–CHCH₂) is 53 ± 1 kcal mol^–1 and Δ_rxn<i>H</i>₂₉₈(C₆H₁₀O → CH₂CH₂ + <i>cis</i>-CH₃CO–CHCH₂) is 41 ± 1 kcal mol^–1. At 1200 K, the products of cyclohexanone pyrolysis were found to be C₆H₉OH, CH₂C­(OH)–CHCH₂, MVK, CH₂CHCH₂, CO, CH₂CO, CH₃, CH₂CCH₂, CH₂CH–CHCH₂, CH₂CHCH₂CH₃, CH₂CH₂, and HCCH

Tabletop Femtosecond VUV Photoionization and PEPICO Detection of Microreactor Pyrolysis Products

We report the combination of tabletop vacuum ultraviolet photoionization with photoion–photoelectron coincidence spectroscopy for sensitive, isomer-specific detection of nascent products from a pyrolysis microreactor. Results on several molecules demonstrate two essential capabilities that are very straightforward to implement: the ability to differentiate isomers and the ability to distinguish thermal products from dissociative ionization. Here, vacuum ultraviolet light is derived from a commercial tabletop femtosecond laser system, allowing data to be collected at 10 kHz; this high repetition rate is critical for coincidence techniques. The photoion–photoelectron coincidence spectrometer uses the momentum of the ion to identify dissociative ionization events and coincidence techniques to provide a photoelectron spectrum specific to each mass, which is used to distinguish different isomers. We have used this spectrometer to detect the pyrolysis products that result from the thermal cracking of acetaldehyde, cyclohexene, and 2-butanol. The photoion–photoelectron spectrometer can detect and identify organic radicals and reactive intermediates that result from pyrolysis. Direct comparison of laboratory and synchrotron data illustrates the advantages and potential of this approach

Pyrolysis of the Simplest Carbohydrate, Glycolaldehyde (CHO−CH₂OH), and Glyoxal in a Heated Microreactor

Both glycolaldehyde and glyoxal were pyrolyzed in a set of flash-pyrolysis microreactors. The pyrolysis products resulting from CHO–CH₂OH and HCO–CHO were detected and identified by vacuum ultraviolet (VUV) photoionization mass spectrometry. Complementary product identification was provided by argon matrix infrared absorption spectroscopy. Pyrolysis pressures in the microreactor were about 100 Torr, and contact times with the microreactors were roughly 100 μs. At 1200 K, the products of glycolaldehyde pyrolysis are H atoms, CO, CH₂O, CH₂CO, and HCO–CHO. Thermal decomposition of HCO–CHO was studied with pulsed 118.2 nm photoionization mass spectrometry and matrix infrared absorption. Under these conditions, glyoxal undergoes pyrolysis to H atoms and CO. Tunable VUV photoionization mass spectrometry provides a lower bound for the ionization energy (IE)­(CHO–CH₂OH) ≥ 9.95 ± 0.05 eV. The gas-phase heat of formation of glycolaldehyde was established by a sequence of calorimetric experiments. The experimental result is Δ_f<i>H</i>₂₉₈(CHO–CH₂OH) = −75.8 ± 1.3 kcal mol^–1. Fully ab initio, coupled cluster calculations predict Δ_f<i>H</i>₀(CHO–CH₂OH) of −73.1 ± 0.5 kcal mol^–1 and Δ_f<i>H</i>₂₉₈(CHO–CH₂OH) of −76.1 ± 0.5 kcal mol^–1. The coupled-cluster singles doubles and noniterative triples correction calculations also lead to a revision of the geometry of CHO–CH₂OH. We find that the O–H bond length differs substantially from earlier experimental estimates, due to unusual zero-point contributions to the moments of inertia

Thermal Decompositions of the Lignin Model Compounds: Salicylaldehyde and Catechol

The nascent steps in the pyrolysis of the lignin components salicylaldehyde (<i>o</i>-HOC₆H₄CHO) and catechol (<i>o</i>-HOC₆H₄OH) were studied in a set of heated microreactors. The microreactors are small (roughly 1 mm ID × 3 cm long); transit times through the reactors are about 100 μs. Temperatures in the microreactors can be as high as 1600 K, and pressures are typically a few hundred torr. The products of pyrolysis are identified by a combination of photoionization mass spectrometry, photoelectron photoion concidence mass spectroscopy, and matrix isolation infrared spectroscopy. The main pathway by which salicylaldehyde decomposes is a concerted fragmentation: <i>o</i>-HOC₆H₄CHO (+ M) → H₂ + CO + C₅H₄CO (fulveneketene). At temperatures above 1300 K, fulveneketene loses CO to yield a mixture of HCC–­CC–CH₃, HCC–­CH₂–CCH, and HCC–­CHCCH₂. These alkynes decompose to a mixture of radicals (HCC–­CC–CH₂ and HCC–CH–­CCH) and H atoms. H-atom chain reactions convert salicylaldehyde to phenol: <i>o</i>-HOC₆H₄CHO + H → C₆H₅OH + CO + H. Catechol has similar chemistry to salicylaldehyde. Electrocyclic fragmentation produces water and fulveneketene: <i>o</i>-HOC₆H₄OH (+ M) → H₂O + C₅H₄CO. These findings have implications for the pyrolysis of lignin itself