Complex organic molecules, the hallmark of terrestrial life, are increasingly detected in exotic environments throughout the universe. Our studies probe the ion chemistry of these biomolecules. We report gas-phase reaction rate constants for five deprotonated nucleobases (adenine, cytosine, guanine, thymine, and uracil) reacting with the atomic species H, N, and O. Hydrogen atoms react at moderate rates via associative electron detachment. Oxygen atom reactions occur more rapidly, generating complex product distributions; reaction pathways include associative electron detachment, substitution of the hydrogen atom by an oxygen atom, and generation of OCN-. Nitrogen atoms do not react with the nucleobase anions. The reaction thermodynamics were investigated computationally, and reported product channels are exothermic. Many of the proposed products have been observed in various astrochemical environments. These reactions provide insight into chemical processes that may occur at the boundaries between diffuse and dense interstellar clouds and in complex extraterrestrial ionospheres.
The anionic products following (H + H+) abstraction from o-, m-, and p-methylphenol (cresol) are investigated using flowing afterglow-selected ion flow tube (FA-SIFT) mass spectrometry and anion photoelectron spectroscopy (PES). The PES of the multiple anion isomers formed in this reaction are reported, including those for the most abundant isomers, o-, m- and p-methylenephenoxide distonic radical anions. The electron affinity (EA) of the ground triplet electronic state of neutral m-methylenephenoxyl diradical was measured to be 2.227 ± 0.008 eV. However, the ground singlet electronic states of o- and p-methylenephenoxyl were found to be significantly stabilized by their resonance forms as a substituted cyclohexadienone, resulting in measured EAs of 1.217 ± 0.012 and 1.096 ± 0.007 eV, respectively. Upon electron photodetachment, the resulting neutral molecules were shown to have Franck-Condon active ring distortion vibrational modes with measured frequencies of 570 ± 180 and 450 ± 80 cm-1 for the ortho and para isomers, respectively. Photodetachment to excited electronic states was also investigated for all isomers, where similar vibrational modes were found to be Franck-Condon active, and singlet-triplet splittings are reported. The thermochemistry of these molecules was investigated using FA-SIFT combined with the acid bracketing technique to yield values of 341.4 ± 4.3, 349.1 ± 3.0, and 341.4 ± 4.3 kcal mol-1 for the o-, m-, and p-methylenephenol radicals, respectively. Construction of a thermodynamic cycle allowed for an experimental determination of the bond dissociation energy of the O-H bond of m-methylenephenol radical to be 86 ± 4 kcal mol-1, while this bond is significantly weaker for the ortho and para isomers at 55 ± 5 and 52 ± 5 kcal mol-1, respectively. Additional EAs and vibrational frequencies are reported for several methylphenyloxyl diradical isomers, the negative ions of which are also formed by the reaction of cresol with O-.
We report the photoelectron spectrum of the aminomethoxide anion (H2C(NH2)O-). The electron affinity (EA) of the aminomethoxy radical is determined to be 1.944(1) eV. Transitions to the ground (XÌ 2Aâ³) and first excited (Ã 2A') electronic states of aminomethoxy are observed, with the term energy measured to be T0(Ã â XÌ) = 0.085(1) eV. A long vibrational progression is observed for the transition to the ground XÌ 2Aâ³ electronic state of aminomethoxy, primarily consisting of OCN bending and HNH wagging vibrations, leading to the assignment of these two fundamental vibrational frequencies of H2C(NH2)O· XÌ 2Aâ³. The gas-phase acidity of aminomethanol is calculated at the G4 level of theory to be Δacid H0Ko = 374.0 kcal mol-1, which, when combined with the experimental EA of aminomethoxy in a thermochemical cycle, provides a determination of the O-H bond dissociation energy, D0(H2C(NH2)O-H) as 106(2) kcal mol-1. Comparisons of the EAs and T0(Ã â XÌ) for the aminomethoxy, methoxy, ethoxy, and hydroxymethoxy radicals provides insight into how the substituent group affects the electronic structure of singly substituted alkoxy radicals.
Anion photoelectron spectra of deprotonated indole have been obtained utilizing several photon energies. The slow electron velocity-map imaging spectrum of indolide allows for the determination of the electron affinity (EA) of indolyl, 2.4315 ± 0.0017 eV. The equilibrium geometry of indolide was shown to minimally distort upon photodetachment with only ring distortion vibrational modes of A' symmetry becoming significantly excited. Photoelectron spectra of indolide accessing the electronic ground state of indolyl displayed a photon energy dependence due to electron autodetachment. Combining the EA of indolyl with the previous work studying the dissociation energy of H-indolyl allows for a new independent measure of ΔacidH0KoN-Hindole ≤ 348.7 kcal/mol, which improves the previous measurement of the gas phase acidity. The anion photoelectron spectrum of deprotonated indoline consisted of a featureless broad band extending from â¼1.3 eV to 1.7 eV electron binding energy. The congested nature of the spectrum is likely due to the presence of multiple isomers of deprotonated indoline, including ring-opened structures.
Vinylidene-acetylene isomerization is the prototypical example of a 1,2-hydrogen shift, one of the most important classes of isomerization reactions in organic chemistry. This reaction was investigated with quantum state specificity by high-resolution photoelectron spectroscopy of the vinylidene anions H2CC- and D2CC- and quantum dynamics calculations. Peaks in the photoelectron spectra are considerably narrower than in previous work and reveal subtleties in the isomerization dynamics of neutral vinylidene, as well as vibronic coupling with an excited state of vinylidene. Comparison with theory permits assignment of most spectral features to eigenstates dominated by vinylidene character. However, excitation of the ν6 in-plane rocking mode in H2CC results in appreciable tunneling-facilitated mixing with highly vibrationally excited states of acetylene, leading to broadening and/or spectral fine structure that is largely suppressed for analogous vibrational levels of D2CC.
Anion photoelectron spectra of the thiazate (NSO-) and thionitrite (SNO-) isomers are reported. The NSO- photoelectron spectrum showed several well-resolved vibronic transitions from the anion to the NSO radical neutral. The electron affinity of NSO was determined to be 3.113(1) eV. The fundamental vibrational frequencies of NSO were measured and unambiguously assigned to be 1202(6) cm-1 (ν1, asymmetric stretch), 1010(10) cm-1 (ν2, symmetric stretch), and 300(7) cm-1 (ν3, bend). From the presence of vibrational hot band transitions, the fundamental vibrational frequencies of the NSO- anion were also measured: 1280(30) cm-1 (ν1, asymmetric stretch), 990(20) cm-1 (ν2, symmetric stretch), and 480(10) cm-1 (ν3, bend). Combined with the previously measured ΔacidH298 Ko(HNSO), D0(H-NSO) was found to be 102(5) kcal/mol. Unlike the results from NSO-, the SNO- photoelectron spectrum was broad with little structure, indicative of a large geometry change between the anion and neutral radical. In addition to the spectrally congested spectrum, there was evidence of a competition between photodetachment from SNO- and SNO- photodissociation to form S- + NO. Quantum chemical calculations were used to aid in the interpretation of the experimental data and agree well with the observed photoelectron spectra, particularly for the NSO- isomer.
The anion photoelectron spectra of ortho-, meta-, and para-methylphenoxide, as well as methyl deprotonated meta-methylphenol, were measured. Using the Slow Electron Velocity Map Imaging technique, the Electron Affinities (EAs) of the o-, m-, and p-methylphenoxyl radicals were measured as follows: 2.1991±0.0014, 2.2177±0.0014, and 2.1199±0.0014 eV, respectively. The EA of m-methylenephenol was also obtained, 1.024±0.008 eV. In all four cases, the dominant vibrational progressions observed are due to several ring distortion vibrational normal modes that were activated upon photodetachment, leading to vibrational progressions spaced by â¼500 cm-1. Using the methylphenol O-H bond dissociation energies reported by King et al. and revised by Karsili et al., a thermodynamic cycle was constructed and the acidities of the methylphenol isomers were determined as follows: ΔacidH298K0=348.39±0.25, 348.82±0.25, 350.08±0.25, and 349.60±0.25 kcal/mol for cis-ortho-, trans-ortho-, m-, and p-methylphenol, respectively. The excitation energies for the ground doublet state to the lowest excited doublet state electronic transition in o-, m-, and p-methylphenoxyl were also measured as follows: 1.029±0.009, 0.962±0.002, and 1.029±0.009 eV, respectively. In the photoelectron spectra of the neutral excited states, C-O stretching modes were excited in addition to ring distortion modes. Electron autodetachment was observed in the cases of both m- and p-methylphenoxide, with the para isomer showing a lower photon energy onset for this phenomenon.
We report the negative ion photoelectron spectroscopy of the hydroxymethoxide anion, H2C(OH)O-. The photoelectron spectra show that 3.49 eV photodetachment produces two distinct electronic states of the neutral hydroxymethoxy radical (H2C(OH)Oâ ). The H2C(OH)Oâ ground state (XÌ 2A) photoelectron spectrum exhibits a vibrational progression consisting primarily of the OCO symmetric and asymmetric stretches, the OCO bend, as well as combination bands involving these modes with other, lower frequency modes. A high-resolution photoelectron spectrum aids in the assignment of several vibrational frequencies of the neutral H2C(OH)Oâ radical, including an experimental determination of the H2C(OH)Oâ 2ν12 overtone of the H-OCO torsional vibration as 220(10) cm-1. The electron affinity of H2C(OH)Oâ is determined to be 2.220(2) eV. The low-lying à 2A excited state is also observed, with a spectrum that peaks â¼0.8 eV above the XÌ 2A state origin. The à 2A state photoelectron spectrum is a broad, partially resolved band. Quantum chemical calculations and photoelectron simulations aid in the interpretation of the photoelectron spectra. In addition, the gas phase acidity of methanediol is calculated to be 366(2) kcal mol-1, which results in an OH bond dissociation energy, D0(H2C(OH)O-H), of 104(2) kcal mol-1, using the experimentally determined electron affinity of the hydroxymethoxy radical.
Dicyanamide [N(CN)2(-)] is a common anionic component of ionic liquids, several of which have shown hypergolic reactivity upon mixing with white-fuming nitric acid. In this study, we explore the thermochemistry of dicyanamide and its reactivity with nitric acid and other molecules to gain insight into the initial stages of the hypergolic phenomenon. We have developed and utilized an electrospray ion source for our selected ion flow tube (SIFT) to generate the dicyanamide anion. We have explored the general reactivity of this ion with several neutral molecules and atoms. Dicyanamide does not show reactivity with O2, H2SO4, H2O2, DBr, HCl, NH3, N2O, SO2, COS, CO2, CH3OH, H2O, CH4, N2, CF4, or SF6 (k < 1 × 10(-12) cm(3)/s); moreover, dicyanamide does not react with N atom, O atom, or electronically excited molecular oxygen (k < 5 × 10(-12) cm(3)/s), and our previous studies showed no reactivity with H atom. However, at 0.45 Torr helium, we observe the adduct of dicyanamide with nitric acid with an effective bimolecular rate constant of 2.7 × 10(-10) cm(3)/s. Intrinsically, dicyanamide is a very stable anion in the gas phase, as illustrated by its lack of reactivity, high electron-binding energy, and low proton affinity. The lack of reactivity of dicyanamide with H2SO4 gives an upper limit for the gas-phase deprotonation enthalpy of the parent compound (HNCNCN; <310 ± 3 kcal/mol). This limit is in agreement with theoretical calculations at the MP2/6-311++G(d,p) level of theory, finding that ΔH298â¯K(HNCNCN) = 308.5 kcal/mol. Dicyanamide has two different proton acceptor sites. Experimental and computational results indicate that it is lower in energy to protonate the terminal nitrile nitrogen than the central nitrogen. Although proton transfer to dicyanamide was not observed for any of the acidic molecules investigated here, the calculations on dicyanamide with one to three nitric acid molecules reveal that higher-order solvation can favor exothermic proton transfer. Furthermore, the formation of 1,5-dinitrobiuret, proposed to be the key intermediate during the hypergolic ignition of dicyanamide ionic liquids with nitric acid, is investigated by calculation of the reaction coordinate. Our results suggest that solvation dynamics of dicyanamide with nitric acid play an important role in hypergolic ignition and the interactions at the droplet/condensed-phase surface between the two hypergolic liquids are very important. Moreover, dicyanamide exists in the atmosphere of Saturn's moon, Titan; the intrinsic stability of dicyanamide strongly suggests that it may exist in molecular clouds of the interstellar medium, especially in regions where other stable carbon-nitrogen anions have been detected.
We report photoelectron spectra of cis-HONO(-) formed from an association reaction of OH(-) and NO in a pulsed, plasma-entrainment ion source. The experimental data are assigned to the cis-HONO(-) isomer, which is predicted to be the global minimum on the anion potential energy surface. We do not find evidence for a significant contribution from trans-HONO(-). Electron photodetachment of cis-HONO(-) with 1613, 1064, 532, 355, and 301 nm photons accesses the ground XÌ (1)A' (S0) and excited ã (3)Aâ³ (T1) states of neutral HONO. The photoelectron spectrum resulting from detachment forming cis-HONO (S0) exhibits a long vibrational progression, dominated by overtones and combination bands involving the central O-N stretching and ONO bending vibrations. This indicates that there is a significant change in the central O-N bond length between cis-HONO(-) and cis-HONO (S0). The electron affinity (EA) of cis-HONO is determined to be 0.356(8) eV. We also report the dissociation energy (D0) of cis-HONO(-), forming OH(-) + NO, as 0.594(9) eV, which is a factor of 4 decrease in the central O-N bond strength compared to neutral cis-HONO. The T1 state of cis-HONO is shown to be â¼2.3 eV higher in energy than cis-HONO (S0). Electron photodetachment to form cis-HONO (T1) accesses a transition state along the HO-NO bond dissociation coordinate. The resulting photoelectron spectrum exhibits broad peaks spaced by the terminal NâO stretching frequency. Electronic structure calculations and photoelectron spectrum simulations reported here show very good agreement with the experimental data.
We report high-resolution photoelectron spectra of the simplest carbanions, CH3â» and CD3â». The vibrationally resolved spectra are dominated by a long progression in the umbrella mode (ν2) of ËCH3 and ËCD3, indicating a transition from a pyramidal C(3v) anion to the planar D(3h) methyl radical. Analysis of the spectra provides electron affinities of ËCH3 (0.093(3) eV) and ËCD3 (0.082(4) eV). These results enable improved determination of the corresponding gas-phase acidities: Δ(acid)H(0K)°(CH4) = 414.79(6) kcal/mol and Δ(acid)H(0K)°(CD4) = 417.58(8) kcal/mol. On the basis of the photoelectron anisotropy distribution, the electron is photodetached from an orbital with predominant p-character, consistent with the sp³-hybridized orbital picture of the pyramidal anion. The double-well potential energy surface along the umbrella inversion coordinate leads to a splitting of the vibrational energy levels of the umbrella mode. The inversion splittings of CH3â» and CD3â» are 21(5) and 6(4) cm⻹, respectively, and the corresponding anion umbrella vibrational frequencies are 444(13) and 373(12) cm⻹, respectively. Quantum mechanical calculations reported herein show good agreement with the experimental data and provide insight regarding the electronic potential energy surface of CH3â».
A novel pulsed anion source has been developed, using plasma entrainment into a supersonic expansion. A pulsed discharge source perpendicular to the main gas expansion greatly reduces unwanted "heating" of the main expansion, a major setback in many pulsed anion sources in use today. The design principles and construction information are described and several examples demonstrate the range of applicability of this anion source. Large OH(-)(Ar)n clusters can be generated, with over 40 Ar solvating OH(-). The solvation energy of OH(-)(Ar)n, where n = 1-3, 7, 12, and 18, is derived from photoelectron spectroscopy and shows that by n = 12-18, each Ar is bound by about 10 meV. In addition, cis- and trans- HOCO(-) are generated through rational anion synthesis (OH(-) + CO + M â HOCO(-) + M) and the photoelectron spectra compared with previous results. These results, along with several further proof-of-principle experiments on solvation and transient anion synthesis, demonstrate the ability of this source to efficiently produce cold anions. With modifications to two standard General Valve assemblies and very little maintenance, this anion source provides a versatile and straightforward addition to a wide array of experiments.
Making use of the extended kinetic method and the alternative method for data analysis, we have experimentally determined ΔH°acid (kcal/mol) for six mononitrated azole species (2-nitropyrrole = 337.0, 3-nitropyrrole = 335.8, 3-nitropyrazole = 330.5, 4-nitropyrazole = 329.5, 2-nitroimidazole = 327.4, and 4-nitroimidazole = 325.0). We report an absolute uncertainty of ±2.2 kcal/mol that arises from the uncertainties of the reference acids; the relative values are known within 0.4 kcal/mol. Combining these experimental ΔH°acid values with ΔS°acid values calculated at the B3LYP/aug-cc-pVTZ level of theory, we report ΔG°acid (kcal/mol) for the nitroazoles (2-nitropyrrole = 329.4, 3-nitropyrrole = 328.4, 3-nitropyrazole = 323.1, 4-nitropyrazole = 322.0, 2-nitroimidazole = 319.7, and 4-nitroimidazole = 317.6); the absolute uncertainties are ±2.4 kcal/mol. In addition to the experimental studies, we have computationally investigated the gas-phase acidities and electron affinities of the azoles in this work, as well as higher-order aza- and dinitro-substituted azoles. We discuss trends in the stabilities of the deprotonated azoles based on aza substitution and nitro group placement. 4-Nitroimidazole has already found use as the anionic component in ionic liquids, and we propose that the additional nitrated azolate ions are potential candidates for the anionic component of ionic liquids.
Azoles/chemistry , Gases/chemistry , Nitrates/chemistry , Computer Simulation , Dimerization , Hydrogen/chemistry , Kinetics , Molecular Structure , Protons
Photofragmentation of ozonide solvated in water clusters, O3(-)(H2O)n, n ≤ 16, has been studied as a function of photon energy as well as the degree of solvation. Using mass selection, the effect of the presence of the solvent molecule on the O3(-) photodissociation process is assessed one solvent molecule at a time. The O3(-) acts as a visible light chromophore within the water cluster, namely the O3(-)(H2O) total photodissociation cross-section exhibits generally the same photon energy dependence as isolated O3(-) throughout the visible wavelength range studied (430-620 nm). With the addition of a single solvent molecule, new photodissociation pathways are opened, including the production of recombined O3(-). As the degree of solvation of the parent anion increases, recombination to O3(-)-based products accounts for close to 40% of photoproducts by n = 16. The remainder of the photoproducts exist as O(-)-based; no O2(-)-based products are observed. Upper bounds on the O3(-) solvation energy (530 meV) and the O(-)-OO bond dissociation energy in the cluster (1.06 eV) are derived.
The photofragmentation dynamics of BrCN(-) in the 270-355 nm and the 430-600 nm wavelength regions is explored both experimentally and theoretically. In the case of excitation between 430 nm and 600 nm, it is found that the molecular ion accesses two dissociation channels with a measured 60:40 branching ratio that is nearly constant over this range of photon energies. The dominant product channel corresponds to Br(-) + CN, while the second channel correlates to spin-orbit excited Br(*) with CN(-). A larger wavelength dependence of the branching ratio is observed at shorter wavelengths, where the fraction of Br(-) based products ranges from 80% to 95% at 355 nm and 270 nm, respectively. These branching ratios are reproduced and the mechanisms are explored by quantum dynamics calculations based on ground and excited state potential energy surfaces for BrCN(-), evaluated at the SO-MRCISD level of theory. It is found that the electronic states that correlate to the two observed product channels are coupled through the spin-orbit terms in the electronic Hamiltonian. The strength of this coupling displays a strong dependence on the Br-CN angle. Specifically, after promotion to the excited state that is energetically accessible with 430-600 nm photons, it is found that when the wave packet accesses Br-CN separations of between 4 Å and 6 Å, predominantly the Br(-) + CN products are formed when the Br-CN angle is smaller than 120°. For larger values of the Br-CN angle, the Br(*) + CN(-) channel dominates. At the shorter wavelength excitation, the dynamics is complicated by a pair of states that correlate to electronically excited CN(*) + Br(-) products that borrow oscillator strength from the bright state, leading to an increase in the amount of Br(-) relative to CN(-). The implications of these findings are discussed and compared to the experimentally measured product branching ratios for the photodissociation of BrCN(-).
We use gas-phase negative ion photoelectron spectroscopy to study the quasilinear carbene propargylene, HCCCH, and its isotopologue DCCCD. Photodetachment from HCCCH affords the XÌ(3B) ground state of HCCCH and its ã(1A), bÌ (1B), dÌ(1A2), and BÌ(3A2) excited states. Extended, negatively anharmonic vibrational progressions in the XÌ(3B) ground state and the open-shell singlet bÌ (1B) state arise from the change in geometry between the anion and the neutral states and complicate the assignment of the origin peak. The geometry change arising from electron photodetachment results in excitation of the ν4 symmetric CCH bending mode, with a measured fundamental frequency of 363 ± 57 cm(1) in the XÌ(3B) state. Our calculated harmonic frequency for this mode is 359 cm(1). The FranckCondon envelope of this progression cannot be reproduced within the harmonic approximation. The spectra of the ã(1A), dÌ(1A2), and BÌ(3A2) states are each characterized by a short vibrational progression and a prominent origin peak, establishing that the geometries of the anion and these neutral states are similar. Through comparison of the HCCCH and DCCCD photoelectron spectra, we measure the electron affinity of HCCCH to be 1.156 ± (0.095)(0.010) eV, with a singlettriplet splitting between the XÌ(3B) and the ã(1A) states of ΔEST = 0.500 ± (0.01)(0.10) eV (11.5 ± (0.2)(2.3) kcal/mol). Experimental term energies of the higher excited states are T0 [bÌ(1B)] = 0.94 ± (0.20)(0.22) eV, T0 [dÌ(1A2)] = 3.30 ± (0.02)(0.10) eV, T0 [BÌ(3A2)] = 3.58 ± (0.02)(0.10) eV. The photoelectron angular distributions show significant π character in all the frontier molecular orbitals, with additional σ character in orbitals that create the XÌ(3B) and bÌ(1B) states upon electron detachment. These results are consistent with a quasilinear, nonplanar, doubly allylic structure of XÌ(3B) HCCCH with both diradical and carbene character.
Photodissociation of ICN(-)(CO2)n, n = 0-18, with 500-nm excitation is investigated using a dual time-of-flight mass spectrometer. Photoabsorption to the (2)Π(1/2) state is detected using ionic-photoproduct action spectroscopy; the maximum absorption occurs around 490 nm. Ionic-photoproduct distributions were determined for ICN(-)(CO2)n at 500 nm. Following photodissociation of bare ICN(-) via 430-650 nm excitation, a small fraction of CN(-) is produced, suggesting that nonadiabatic effects play a role in the photodissociation of this simple anion. Electronic structure calculations, carried out at the MR-SO-CISD level of theory, were used to evaluate the potential-energy surfaces for the ground and excited states of ICN(-). Analysis of the electronic structure supports the presence of nonadiabatic effects in the photodissociation dynamics. For n ≥ 2, the major ionic photoproduct has a mass corresponding to either partially solvated CN(-) or partially solvated [NCCO2](-).
The ultraviolet (UV) photodissociation of mass-selected ICN(-)Ar(n) and ICN(-)(CO2)n clusters (n = 0-5) is studied using a secondary reflectron mass spectrometer. Relative photodissociation cross sections of bare ICN(-) show the dominance of the I(-) photoproduct from 270 to 355 nm, the entire wavelength range studied. UV excitation populates both the (2)Σ(+) state that produces I* + CN(-) and the (2)Π states that produce I(-) + CN*. While the excited (2)Π states directly produce I(-), excitation to the (2)Σ(+) state also produces some I(-) product via nonadiabatic transitions to the (2)Π(1/2) state, which produces I(-) + CN. Partial solvation of the anion by Ar atoms or CO2 molecules alters the UV-branching percentages between the various dissociation channels: I* + CN(-) and I(-) + CN or I(-) + CN*. In addition, solvation by two or more Ar atoms or three or more CO2 molecules results in recombination, reforming ICN(-). Examination of the potential surfaces and transition moments in combination with the results of quantum dynamics calculations performed on the relevant excited states assist in the analysis of the experimental results.
This contribution is very much a personal history of a journey through the wonderful world of anion chemistry, and a tale of how advances in laser technologies, theoretical methods, and computational capabilities continuously enabled advances in our understanding. It is a story of the excitement and joy that come from the opportunity to add to the fabric of science, and to do so by working as a group of excited explorers with common goals. The participants in this journey include me, my students and postdoctoral associates, my collaborators, and our many generous colleagues. It all happened, in the words of the Beatles, "with a little help from my friends." Actually, it was so much more than a little help!
The negative ion chemistry of five azine molecules has been investigated using the combined experimental techniques of negative ion photoelectron spectroscopy to obtain electron affinities (EA) and tandem flowing afterglow-selected ion tube (FA-SIFT) mass spectrometry to obtain deprotonation enthalpies (Δ(acid)H(298)). The measured Δ(acid)H(298) for the most acidic site of each azine species is combined with the EA of the corresponding radical in a thermochemical cycle to determine the corresponding C-H bond dissociation energy (BDE). The site-specific C-H BDE values of pyridine, 1,2-diazine, 1,3-diazine, 1,4-diazine, and 1,3,5-triazine are 110.4 ± 2.0, 111.3 ± 0.7, 113.4 ± 0.7, 107.5 ± 0.4, and 107.8 ± 0.7 kcal mol(-1), respectively. The application of complementary experimental methods, along with quantum chemical calculations, to a series of nitrogen-substituted azines sheds light on the influence of nitrogen atom substitution on the strength of C-H bonds in six-membered rings.
