Negative Ion Photoelectron Spectra of Deprotonated Benzonitrile Isomers via Computation of Franck–Condon Factors

Abstract

Negative ion photoelectron spectra of ortho (o-), meta (m-), and para (p-) deprotonated benzonitrile (o-, m-, p-C 6 H 4 (CN) − ) isomers as well as the associated thermochemical values corresponding to deprotonation at o-, m-, and p-positions in C 6 H 5 (CN) are presented. Quantum mechanical results based on the density functional theory (DFT) utilizing the aug-cc-pVQZ basis set indicate that the o-, m-, p-C 6 H 4 (CN) • radicals have electron affinity values (EAs) of 1.901, 1.778, and 1.789 eV, respectively. The computed Franck−Condon (FC) factors give rise to o-, m-, and p-C 6 H 4 (CN) − negative ion spectra with FC active ring distortion vibrational modes with harmonic vibrational frequencies of ∼450, 760, and 1000 cm −1 as the dominant vibrational progressions. Deprotonation at the o-, m-, and ppositions in C 6 H 5 (CN) results in calculated gas-phase acidity values (Δ acid H 298K o ) of 383.9, 385.7, and 385.3 kcal mol −1 , respectively. The calculated Δ acid H 298K o is in close agreement with the previously reported high-pressure mass spectrometry experimental value of 383.4.0 ± 4.4 kcal mol −1 . The computed Δ acid H 298K o and EAs are utilized to estimate the bond dissociation energy (DH 298 (H-C 6 H 4 CN)) associated with the formation o-, m-, and p-C 6 H 4 (CN) • using the negative ion thermochemical cycle: DH 298 (C 6 H 5 CN) = Δ acid H 298K o (H-C 6 H 4 (CN) + EA (C 6 H 5 CN) • − IP(H). The respective values of DH 298 (H-C 6 H 4 CN) corresponding to the formation of ortho, meta, and para C 6 H 4 (CN) radicals are 114.15, 113.11, and 113.51 kcal mol −1 .

Photoelectron Spectra of Hot Polyatomic Ions: A Statistical Treatment of Phenide

“…For example, a Franck–Condon (FC) analysis − accounting for all vibrational states that contribute significantly to the partition function of phenide at T ≈ 1000 K would take millennia of computing time (vide infra). In contrast, similar calculations in the often-assumed 0–300 K range require few initial states to be considered and thus avoid the above limitation. − …”

From Benzonitrile to Dicyanobenzenes: The Effect of an Additional CN Group on the Thermochemistry and Negative Ion Photoelectron Spectra of Dicyanobenzene Radical Anions

“…To the best of our knowledge, this paper is the first to report positional gas-phase acidity values of the three DCNB isomers. From our recent study where we reported site-specific, bond-dissociation energies, electron affinities and gas-phase acidities in BZN, it is clear that the substitution of a H atom in BZN with a CN group to form DCNB results in a significant decrease in Δ acid H 298 K o values in DCNB isomers, implying that the protons in DCNB are more acidic than the ones in BZN. From Table , an ∼60 kJ mol –1 decrease in the calculated Δ acid H 298 K o of DCNB is seen compared to BZN, with slight differences of ∼10 kJ mol –1 in regard to positional acidities among the DCNB isomers.…”

“…To the best of our knowledge, this paper is the first to report positional gas-phase acidity values of the three DCNB isomers. From our recent study where we reported site-specific, bond-dissociation energies, electron affinities and gas-phase acidities in BZN, 71 72 The higher acidity of linear and cyclic dicyano protons than their cyano counterparts reflects the extent to which an additional CN group stabilizes the unpaired electron in the respective radicals via increased π resonance interaction. This gas-phase acidity trend is also manifested in the EAs of the respective radicals, since higher gas-phase acidities translate into greater radical stability.…”

Predicted Negative Ion Photoelectron Spectra of 1-, 2-, and 9-Cyanoanthracene Radical Anions and Computed Thermochemical Values of the Three Cyanoanthracene Isomers

“…The present study involves computing the negative ion photoelectron spectra of ACN anions and the subsequent determination of the EAs associated with the deprotonated ACN radical species at different ring deprotonation sites. Therefore, a gas-phase acidity definition associated with the deprotonation enthalpy (Δ acid H 298K ° ) is adopted, similarly to previous studies involving benzonitrile and dicyanobenzenes . Finally, our computed bond dissociation enthalpies were then compared with the ones predicted by the machine learning (ML) prediction interface tool that is available online at .…”

“…This is contrary to deprotonation in benzonitrile where deprotonation at the position adjacent to the CN group (ortho position) results in the most stable anion. 62,88 For 1-ACN and 2-CAN, Table 4 shows that the most stable anions (highest EA of the radicals) in each case are the ones closest to the CN group (at position 1 in 2-ACN and position 2 in 1-ACN) while the least stable (lowest EA) anions are the ones where deprotonation occurs farthest from the CN group (position 6 and 7). Nevertheless, it is important to note that deprotonating at positions 6 and 7 in both 1-and 2-ACN isomers result in isomers that differ by only 12 meV.…”