Aromatic Stabilization Energy Research Articles

Computational study on the stability of imidazol-2-ylidenes and imidazolin-2-ylidenes

The dimerization reactions of 12 cyclic diaminocarbenes have been studied using density functional theory (DFT). The activation energies ( E a) and reaction energies ( ΔE) for the dimerizations of imidazol-2-ylidenes are larger than those of imidazolin-2-ylidenes. It was observed that E a of dimerization is approximately proportional to the singlet–triplet energy separation ( ΔE S–T), aromatic stabilization energy (ASE), and ΔE of the carbene. Excellent linear correlation is seen between E a and ASE. Contrary to previous suggestions, we found that 4,5-dichloro substitutions decrease the stability of imidazol(in)-2-ylidenes. Steric effects on E a occur noticeably as isopropyl ( i-Pr) substitutions are introduced to the carbenes.

From localized to delocalized annulenes: how ring strain enhances delocalization in higher annulenes

Strategies to stabilize delocalized structures in higher annulenes have been explored using density functional calculations at the B3LYP/6-31G(d) level. Symmetrically placed methylene bridges in [30]- and [42]annulenes favor delocalized structures (D 6h) over localized (D 3h) ones whereas [54]- and [66]annulenes still prefer localized structures after modification. Aromatic stabilization energy in delocalized structures reaches a constant value of 1.5 kcal/mol per π-electron pair from [54]annulene. Strain is higher in smaller annulenes (D 6h) and decreases as the annulenes become larger. Localized annulenes (D 3h) exhibit much smaller ring strain energy, which steeply falls down for higher annulenes.

Global and Local Aromaticity of Linear and Angular Polyacenes

Ab initio (B3LYP/6-311G∗∗) optimisation of seven linear and five angular polyacenes gave us an opportunity to study aromaticity in terms of HOMA index and its components EN and GEO, the energy of the carbon skeleton of these hydrocarbons was calculated from CC bond lengths. Independently, the Cohen–Benson group additivity values were used to estimate the thermochemical aromatic stabilisation energy (ASE). For individual rings the NICS values were computed and compared with HOMA. An increase of size of both linear and angular polyacenes is associated with a substantial decrease in their aromaticity, with a greater decrease for the linear polyacenes. This is well documented by both HOMA and Cohen–Benson ASE.

Theoretical predictions of the structure, gas-phase acidity and aromaticity of 1,2-diseleno-3,4-dithiosquaric acid

Results of ab initio self-consistent-field (SCF) and density functional theory (DFT) calculations of the gas-phase structure, acidity (free energy of deprotonation, ΔGo), and aromaticity of 1,2-diseleno-3,4-dithiosquaric acid (3,4-dithiohydroxy-3-cyclobutene-1,2-diselenone, H2C4Se2S2) are reported. The global minimum found on the potential energy surface of 1,2-diseleno-3,4-dithiosquaric acid presents a planar conformation. The ZZ isomer was found to have the lowest energy among the three planar conformers and the ZZ and ZE isomers are very close in energy. The optimized geometric parameters exhibit a bond length equalization relative to reference compounds, cyclobutanediselenone, and cyclobutenedithiol. The computed aromatic stabilization energy (ASE) by homodesmotic reaction (Eq 1) is −20.1 kcal/mol (MP2(fu)/6-311+G** //RHF/6-311+G**) and −14.9 kcal/mol (B3LYP//6-311+G**//B3LYP/6-311+G**). The aromaticity of 1,2-diseleno-3,4-dithiosquaric acid is indicated by the calculated diamagnetic susceptibility exaltation (Λ) −17.91 (CSGT(IGAIM)-RHF/6-311+G**//RHF/6-311+G**) and −31.01 (CSGT(IGAIM)-B3LYP/6-311+G**//B3LYP/6-311+G**). Thus, 1,2-diseleno-3,4-dithiosquaric acid fulfils the geometric, energetic and magnetic criteria of aromaticity. The calculated theoretical gas-phase acidity is ΔGo1(298K)=302.7 kcal/mol and ΔGo2(298K)=388.4 kcal/mol. Hence, 1,2-diseleno-3,4-dithiosquaric acid is a stronger acid than squaric acid(3,4-dihydroxy-3-cyclobutene-1,2-dione, H2C4O4).

Ab initio and density functional theory studies of the structure, gas‐phase acidity and aromaticity of tetraselenosquaric acid

AbstractResults of ab initio self‐consistent‐field (SCF) and density functional theory (DFT) calculations of the gas‐phase structure, acidity (free energy of deprotonation, ΔG*) and aromaticity of tetraselenosquaric acid (3, 4‐diselenyl‐3‐cyclobutene‐1,2‐diselenone, H2 C4 Se4)are reported. The global minimum found on the potential energy surface of tetraselenosquaric acid presents a planar conformation. The ZZ isomer was found to have the lowest energy among the three planar conformers and the ZZ and ZE isomers are very dose in energy. The optimized geometric parameters exhibit a bond length equalization relative to reference compounds, cyclobutanediselenone, and cyclobutanediselenol. The computed aromatic stabilization energy (ASE) by homodesmotic reaction is ‐77.4 (MP2(fu)/6–311+G** /RHF/6 ‐ 311 + G** ) and ‐ 54.8 kJ/mol (B3LYP/6 ‐ 311 + G** //B3LYP/6 ‐311 + G**). The aromaticity of tetraselenosquaric acid is indicated by the calculated diamagnetic susceptibility exaltation (A) ‐19.13 (CSGT(IGAIM)‐RHF/6–311 + G**// RHF/6–311 + G** and ‐32.91 (4π·10−6 m−3/mol)(CSGT(I‐GAIM)‐B3LYP/6 ‐ 311 + G* * //B3LYP/6 ‐ 311 + G**). Thus, tetraselenosquaric acid fulfils the geometric, energetic and magnetic criteria of aromaticity. The calculated gas‐phase acidity is ΔG1*(298k) = 1257.7 and ΔG*2 (298k) = 1617.1 kJ/mol. Hence, tetraselenosquaric acid is the strongest acid among the three squaric acids (3, 4‐dihydroxy‐3‐cyclobutene‐1, 2‐dione, H2 C4 3,4‐dithiohydroxy‐3‐cyclobutene‐1,2‐dithione, H2C4 S4, 3, 4‐diselenyl‐3‐cyclobutene‐1,2‐diselenone, H2C4Se4).

The gas-phase acidity and aromaticity of squaric acid: an ab initio and density functional theory study

The conformation, aromaticity and gas-phase acidity (free energy of deprotonation, ΔGo) of squaric acid (3,4-dihydroxy-3-cyclobutene-1,2-dione) were calculated at the SCF and MPn (n=2,3,4), and B3LYP levels using the 6-311G(d,p) and 6-311+G(d,p) basis sets. The global minimum found on the potential energy surface of squaric acid presents a planar conformation. The ZZ isomer was found to be the most stable of the three planar conformers and the ZZ and ZE isomer are very close in energy. The optimized geometrical parameters exhibit a bond length equalization relative to reference molecules, cyclobutandione, and cyclobutenediol. The computed aromatic stabilization energy (ASE) by homodesmotic reaction is −16.3 (MP4(sdtq)/6-311+G(d,p)//RHF/6-311+G(d,p)) and −12.9kcal/mol (B3LYP/6-311+G(d,p)//B3LYP/6-311+G(d,p)). The aromaticity of squaric acid is indicated by the calculated diamagnetic susceptibility exaltation (Λ)−2.07 (CSGT(IGAIM)-RHF/6-311+G(d,p)//RHF/6-311+G(d,p)) and −2.16 (CSGT(IGAIM)-B3LYP/6-311+G(d,p)//B3LYP/6-311+G(d,p)). Thus, squaric acid fulfils the geometrical, energetic and magnetic criteria of aromaticity. The most reliable theoretical gas-phase acidity is ΔG1(298K)o=311.4 and ΔG2(298K)o=421.0kcal/mol.

Ab initio and density functional predictions of the structure, gas-phase acidity and aromaticity of 1,2-dithio-3,4-diselenosquaric acid

Results of ab initio self-consistent-field and density functional theory calculations of the gas-phase structure, acidity (free energy of deprotonation, Δ G 0) and aromaticity of 1,2-dithio-3,4-diselenosquaric acid (3,4-diselenyl-3-cyclobutene-1,2-dithione, H 2C 4S 2Se 2) are reported. The optimised geometrical parameters result in an equalization of the bond lengths compared with the reference compounds cyclobutandithione and cyclobutenediselenol. The computed aromatic stabilization energy and diamagnetic susceptibility exaltation ( Λ) are negative, indicating that 1,2-dithio-3,4-diselenosquaric acid is aromatic. Thus, 1,2-dithio-3,4-diselenosquaric acid fulfils the geometrical, energetic and magnetic criteria of aromaticity.

Does Cr(CO)3 Complexation Reduce the Aromaticity of Benzene?

Ring currents in (C6H6)Cr(CO)3, 1, (cis-1,3-butadiene)Cr(CO)4 (4), (C6H6)2Cr (5), (C4H4)Fe(CO)3 (6), and (cis-1,3-butadiene)Fe(CO)3 (8) have been assessed by σ−π disected nucleus independent chemical shift (NICS) calculations. Shielding contributions from the C−C(π) orbitals to the NICS values reveal that there is no quenching of ring current in the benzene ring of 1 or in dibenzene chromium 5. The previously reported paratropic ring current for 1 is shown to be a consequence of latent aromaticity in 1,3-butadiene chromium tetracarbonyl 4 (one of the molecules used in the magnetic susceptibility equation). NICS values, a diatropic ring current, and a positive aromatic stabilization energy (ASE) all point to this aromaticity. NICS values for cyclobutadiene iron tricarbonyl, 6, show moderately sized diamagnetic shielding above the plane of the four-membered ring. In addition, 6 has a negative magnetic susceptibility exaltation (MSE) (diatropic ring current), quite opposite from the large paratropic current ...

Synthesis of Stable 2-Silanaphthalenes and Their Aromaticity

Stable neutral silaaromatic compounds, 2-silanaphthalenes (1a; R = Tbt, 1b; R = Bbt), were synthesized by taking advantage of extremely bulky and efficient steric protection groups, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt) and 2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl (Bbt). 2-Silanaphthalenes 1a,b were isolated as colorless crystalline compounds, and the structure of Tbt-substituted 1a was determined by X-ray crystallographic analysis. All 1H, 13C, and 29Si NMR signals of the silanaphthalene ring of 1 were in good agreement with those calculated; downfield-shifted 1H signals indicating aromatization were observed. Refined NICS (nucleus-independent chemical shift) calculations which separate the σ and π contributions revealed the presence of comparably large ring current effects in the two rings of 2-silanaphthalene. The aromatic stabilization energies (ASEs) of 1-, 2-, and 9-silanaphthalenes, computed by a novel isomerization method, are almost the same as naphthale...

Modern Molecular Mechanics and ab Initio Calculations on Benzylic and Cyclic Delocalized Cations

Calculations of optimized force field (MMP2 extended to carbocations) and ab initio (MP2/6-31G*) geometries as well as π-electron densities of various benzyl and cyclic delocalized cations agree well. The MMP2 heats of formation reproduce the available experimental values. MMP2 π-resonance energies are consistent with those obtained by isodesmic equations from experimental and ab initio data. When carbon π-charges are lower than 0.2, the influence of phenyl substituents is attenuated. Thus, the triphenylmethyl cation resonance stabilization value (−41.6 kcal/mol average for each phenyl ring) is much less than that of the benzyl cation (−76.4 kcal/mol) and the benzhydryl cation (average stabilization value of −51.4 kcal/mol). MMP2 aromatic stabilization energy estimates of the benzyl and tropylium cations as well as benzene agree well with the assessments of aromaticity by the nucleus independent chemical shift (NICS) criterion, which is based on the magnetic shieldings computed at ring centers. The MMP2 m...

Application of the orbital deletion procedure (ODP) to planar carbocations

An orbital deletion procedure (ODP), which inactivates a critical π orbital and therefore interrupts cyclic conjugation, and the nucleus independent chemical shifts (NICS) have been employed to evaluate the aromaticity of the tropylium, cyclopropenyl and benzyl cations, as well as the antiaromaticity of the singlet cyclopentadienyl cation, compared with their reference cations: 1,3,5-heptatrienyl, allyl, 1,3,5-hexatrienyl-3-carbinyl and 1,3,5-pentadienyl. The aromatic stabilization energies, defined as the difference in the delocalization energies (deduced from ODP) between the cyclic and the acyclic reference cation, are in general agreement with the values from appropriate isodesmic equations.

Synthesis and Properties of Photochromic Diarylethenes with Heterocyclic Aryl Groups

Abstract A new class of thermally irreversible and fatigue resistant photochromic molecules named diarylethenes has been developed. Theoretical consideration based on the state correlation diagrams of electrocyclic reactions revealed that the thermal irreversibility is attained by introducing aryl groups that have low aromatic stabilization energy. According to the molecular design principle thermally irreversible photochromic diarylethenes having heterocyclic five-membered rings, such as furan, thiophene (or benzothiophene), or thiazole rings, have been newly synthesized. Among them, diarylethenes having benzothiophene aryl groups showed an excellent fatigue resistant property. They underwent photochromic cycles more than 1.0 × 104 times. The absorption maxima of the closed-ring forms varied from 425 to 828 nm depending on the aryl groups, substituents of the aryl groups, and the substitution position of the aryl groups to the ethane moiety. The response times of both cyclization and ring-opening reactions were less than 10 ps in solution as well as in a crystalline phase. The chemical and thermal control of photochromic reactivity was achieved by introducing intramolecular hydrogen bonding groups or oligothiophene aryl groups to the diarylethenes. Some of the dithienylperfluorocyclopentene derivatives were found to show photochromic reactivity in the crystalline phase. The reactivity was dependent on the conformation and molecular packing in the crystals.

Bond length alternation and aromaticity in large annulenes

Properties of [4n] and [4n+2]annulenes were studied as a function of n for up to [66]annulene using Hartree–Fock and density functional theory in the generalized gradient approximation (DFT-GGA). In the 4n+2 series a “transition” from delocalized to localized structures occurs at 4n+2=30. Various indices of aromaticity, including NMR chemical shifts, bond localization, and aromatic stabilization energy (ASE) were monitored. π-bond localization occurs not due to a dramatic decrease of ASE as n increases, but rather as a result of a pseudo-Jahn–Teller (PJT) effect that sets in as the HOMO-LUMO gap decreases with increasing size. The NMR measures of aromaticity (difference between inner and outer 1H chemical shielding constants and the nucleus-independent chemical shifts, NICS) are reduced in the localized structures in comparison to the delocalized ones. The gradual nature of this “transition” is also implied by the relatively large values of the NMR measures of aromaticity that approach zero only gradually for larger size annulenes. Therefore intermediate size annulenes, such as [30]annulene are predicted to have a localized structure and aromatic properties at the same time showing the delocalized structure is not a necessary condition to be aromatic.

Aromaticity in X(3)Y(3)H(6) (X = B, Al, Ga; Y = N, P, As), X(3)Z(3)H(3) (Z = O, S, Se), and Phosphazenes. Theoretical Study of the Structures, Energetics, and Magnetic Properties.

A systematic estimation of aromaticity in X(3)Y(3)H(6), 1, (X = B, Al, Ga; Y = N, P, As), P(3)N(3)H(6), 2, and X(3)Z(3)H(3), 3 (Z = O, S, Se), has been conducted using structural, energetic, and magnetic criteria. Estimates based on aromatic stabilization energy (ASE) calculations predict that 1BN (1; X = B, Y = N) and 1BP are equally aromatic. Contrary to this, we have found, from magnetic susceptibility exaltation (MSE) and from the nucleus independent chemical shift (NICS) data at the B3LYP/6-31G level, that 1BN is not aromatic while 1BP is. This emphasizes the fact that energetic and magnetic criteria need not be parallel. On the basis of MSE and NICS values, all 1XP compounds show strong aromatic character; 1XAs are borderline aromatic while 1XN compounds are nonaromatic. Despite being aromatic, all 1XP and 1XAs compounds are found to prefer nonplanar geometries. MSE and NICS criteria can also diverge quite strongly; this has been observed in the X(3)Z(3)H(3) family. MSE values for 3BS, 3BSe, 3AlO, 3AlS, and 3GaS are more than half of the MSE value for benzene, indicating substantial aromatic character. However, NICS estimates point to the contrary; none of the type 3 compounds are aromatic. The problem with the ASE and MSE is that both depend on the choice of the reference systems while NICS, which avoids the need for reference molecules, is impossible to vary experimentally. In spite of this epistemological deficiency of NICS, we find it complementary to the ASE and MSE criteria. Despite the existence of a large number of well-established structures and substantial aromatic stabilization energy, phosphazenes, 2, are not aromatic according to NICS data.

Aromaticity of Annelated Borepins

Significant C−C bond length alternation (Δr = 0.047 A), very low aromatic stabilization energy (ASE = −5.1 kcal/mol), magnetic susceptibility anisotropy (χanis = −83.1 ppm cgs), and exaltation (Λ = −13.8 ppm cgs), as well as the nucleus-independent chemical shifts (NICS(0), −3.7 ppm) demonstrate borepin to be much less aromatic than the tropylium ion. The B3LYP/6-311+G** relative stabilities of annelated borepin positional isomers isoelectronic with azulene (1a) and the benzotropylium cation (1b), e.g., the boraazulene anion (2c > 2b > 2a) and the benzo-fused borepins (5c > 5b > 5a), are in accord with the topological charge stabilization rule. However, the aromaticity orderings (2a > 2b > 2c; 5a > 5c > 5b) are opposite, as deduced from the Δr, χanis, and NICS values. All the aromaticity criteria support 4X > 3X ordering for the heterole-fused borepin positional isomers (3X, 4X; X = NH, S, O), in agreement with their relative stabilities. The NICS(1) values (i.e., computed 1 A above the ring centers) indi...

Ab initio calculations on 1,3,2-diazaphospholes: New heteroaromatic systems

Ab initio calculations on three 1,3,2-diazaphosphole derivatives [(MP2/6-31G(d,p)] gave rise to structural and energetic data that are interpreted in context of aromaticity. The 1,3,2-diazaphospholenium ion is shown to have a degree of aromatic stabilization energy (24.0 kcal/mol) comparable to that of pyrrole. Cyclic delocalization is supported by analysis of computed charge distribution data, natural bond orbital data, bond lengths, and magnetic susceptibility data.

Why Pentaphosphole, P5H, Is Planar in Contrast to Phosphole, (CH)4PH

In contrast to phosphole, (CH)4PH, pentaphosphole, P5H, has a planar C2v minimum. At RMP2(fc)/6-31G* + ZPE(RMP2(fc)/6-31G*//RMP2(fc)/6-31G*) the aromatic stabilization energy (ASE) of P5H (17 kcal/mol) is larger than of (CH)4PH (ASE = 7 kcal/mol) but smaller than of (CH)4NH (25.5 kcal/mol) and N5H (29 kcal/mol). However, pentaphosphole is thermodynamically stable by 58.2 kcal/mol (54.7 kcal/mol, MP4SDTQ(fc)/6-31G*//MP2(fc)/6-31G* + ZPE(RHF/6-31G*)) toward dissociation into PPPH and P2. Other P5H isomers have relative energies of 9 (tricyclic, Cs), 17.4 (bicyclic, Cs), 25.6 (monocyclic, P3PPH, Cs), and 53.5 (open chain, PPPPPH, C1) kcal/mol. Due to its electropositive character, the -PH2 substituent reduces the inversion barrier of tricoordinate phosphorus, ΔEinv(P), from 35 kcal/mol for PH3 to 20.3 kcal/mol for HP(PH2)2 and 16.1 for P(PH2)3. In addition, π conjugation of −PPH substituents reduce ΔEinv(P) further to 13.5 in HP(PPH)2 and 6.1 kcal/mol in P(PPH)3. In contrast, the vinyl groups in HP(CHCH2)2 a...

Besides N(2), What Is the Most Stable Molecule Composed Only of Nitrogen Atoms?

Polynitrogen molecules have been studied systematically at high levels of ab initio and density functional theory (DFT). Besides N(2), the thermodynamically most stable N(n)() molecules, located with the help of a newly developed energy increment system, are all based on pentazole units. The geometric, energetic, and magnetic criteria establish pentazole (2) and its anion (3) to be as aromatic as their isoelectronic analogues, e.g., furan, pyrrole, and the cyclopentadienyl anion. The bond lengths in 2 and 3 are equalized; both have large aromatic stabilization energies (ASE) and also substantial magnetic susceptibility exaltations (Lambda). The C(s)() symmetric azidopentazole (14), a candidate for experimental investigation, is the lowest energy N(8) isomer but is still 196.7 kcal/mol higher in energy than four N(2) molecules. Octaazapentalene (12) with 10 pi electrons also is aromatic. The D(2)(d)() symmetric bispentazole (21) is the lowest energy N(10) minimum but is 260 kcal/mol higher in energy than five N(2) molecules. For strain-free molecules, the average deviation is +/-2.6 kcal/mol between the DFT energies and those based on the increment scheme. The increment scheme also provides estimates of the strain energies of polynitrogen compounds, e.g., tetraazatetrahedrane (8, 48.2 kcal/mol), octaazacubane (11, 192.6 kcal/mol), and N(20) (27, 294.6 kcal/mol), and is useful in searching for new high-energy-high-density materials.

Thermochemical Assessment of the Aromatic and Antiaromatic Characters of the Cyclopropenyl Cation, Cyclopropenyl Anion, and Cyclopropenyl Radical: A High-Level Computational Study

The aromatic stabilization energy of the cyclopropenyl cation (CH)3+ is assessed with G2 theory by calculating its homodesmotic stabilization energy (247.3 kJ mol-1) and by comparing the ionization energies of the cyclopropenyl radical (6.06 eV) and the cyclopropyl radical (8.24 eV). These data indicate substantial stabilization of the two π-electron system in what is considered the archetypal aromatic cation. The calculated enthalpy of formation of the cyclopropenyl cation is 1074.0 kJ mol-1 and agrees with the experimental estimate of 1075 kJ mol-1. The small stabilization energy of the cyclopropenyl radical (37.4 kJ mol-1) suggests that this radical should not be classified as aromatic, in contrast to earlier suggestions. Our G2-calculated enthalpy of formation of the cyclopropenyl radical (ΔHf 298 = 487.4 kJ mol-1) and its ionization energy are different from experimental estimates and suggest that the experimental values may need to be revised. The most stable structure for the cyclopropenyl anion is...

An ab initio study of some heterocyclic 6π-carbenes

Ab initio calculations on a series of 6π-electron heterocyclic ring systems containing O, N, and S atoms provided evidence for aromatic stabilization ranging from 7.6 to 25.5 kcal/mol. The most highly stabilized system was the imidazolyl carbene which had an aromatic stabilization energy comparable to furan and pyrrole. Proton affinity data was consistent with literature pKa measurements of the conjugate acids.