Comparative study of the hypercoordinate ions C7H9+ and C8H9+ by the ab initio/GIAO-CCSD(T) method

A comparative study of the hypercoordinate square-pyramidal carbocations C7H9+ and C8H9+ was performed by the ab initio/GIAO-CCSD(T) method. The structures and 13C NMR chemical shifts of the cations were calculated at the GIAO-CCSD(T)/tzp/dz//MP2/cc-pVTZ level. The bishomo square pyramidal structure 1 was calculated for C7H9+ at the MP2/cc-pVTZ level. The calculated 13C NMR chemical shifts of structure 1 agree extremely well with the experimental values. However, unlike for C7H9+ both the bishomo square pyramidal structure 3 and the trishomocyclopropenium type structure 4 were found to be minima on the potential energy surface of C8H9+. They are very close energetically with cation 3, only 0.7 kcal/mol less stable than cation 4 at the MP2/cc-pVTZ//MP2/cc-pVTZ + ZPE level. Neither structure 3 nor 4 yields NMR spectra that agree with experiment. However, a weighted average of the two reproduces the observed NMR spectrum of C8H9+ (at -80 degrees C) quite well.     

Structural studies of nonclassical cyclobutylmethyl cations by the ab initio method

Ab initio calculations at the MP2/cc-pVTZ level show that the cyclobutylmethyl cation is a nonclassical sigma-delocalized species, which is distinct from the global minimum C2-symmetric cyclopentyl cation (Schleyer, P. v. R.; Carneiro, J. W. de M.; Koch, W.; Raghavachari, K. J. Am. Chem. Soc. 1989, 111, 5475). Relatively lower level DFT calculations, on the other hand, show that the primary cyclobutylmethyl cation spontaneously collapses into the cyclopentyl cation (Prakash, G. K. S.; Reddy, V. P.; Rasul, G.; Casanova, J.; Olah, G. A. J. Am. Chem. Soc. 1998, 120, 13362). The secondary 1-cyclobutylethyl cation is also a nonclassical carbocation, as shown by calculations at the MP2/cc-pVTZ level. Two structures having energy minima are identified for the latter cation on the potential energy surface. The conformer in which the methyl group is in the exo orientation is a global minimum and is favored over the corresponding endo conformer by 1.2 kcal/mol at the MP2/cc-pVTZ//MP2/cc-pVTZ +ZPE level of calculations. The tertiary 1-cyclobutyl-1-methylethyl cation, at this level of calculations, also involves substantial nonclassical sigma-delocalization, showing that the nonclassical stabilization is more important for cyclobutylmethyl cations relative to the cyclopropylmethyl cations. The 13C NMR chemical shifts obtained from GIAO-CCSD(T)/tzp/dz calculations further substantiate the nonclassical structures for these carbocations.     

Ab initio/GIAO-CCSD(T) study of propenoyl (H2C=CH-CO+) and isopentenoyl ((CH3)2C=CH-CO+) cations and their superelectrophilic protonated dications

Structures of superelectrophilic protonated propenoyl (H2C=CH-COH2+) and isopentenoyl ((CH3)2C=CH-COH2+) dications and their parent cations were calculated using ab initio methods at the MP2/6-311+G and MP2/cc-pVTZ levels. Energies were calculated using Gaussian-2 (G2) theory. The alpha-carbon (Calpha) protonated 3 and 7 were found to be the global minima for protonated propenoyl and isopentenoyl dications, respectively. 13C NMR chemical shifts of the cations were also calculated using the GIAO-CCSD(T), GIAO-MP2 and GIAO-SCF methods. 13C NMR chemical shifts of the related tert-butyl cation ((CH3)3C+) and protonated tert-butyl dication ((CH3)2CCH4(2+)) were also computed at the same level to compare and explore the effect of the additional charge in dications.     

Ab Initio/GIAO-CCSD(T) study of bicyclic and related strained olefins. Structures and 13C NMR chemical shifts

Bicyclic and related strained olefins were studied by the ab initio/GIAO-CCSD(T) method. Structures and (13)C NMR chemical shifts of the olefins were calculated using ab initio/GIAO-CCSD(T) method. The delta(13)C of the olefinic carbons of the yet unknown bicyclo[1.1.0]but-1,3-ene 1 and bicyclo[2.1.0]pent-1(4)-ene 2 were computed to be 69.4 and 212.4, respectively, at the GIAO-CCSD(T)/qzp/tzp//MP2/cc-pVTZ level. The delta(13)C of the olefinic carbons of the intriguing (larger and also yet unknown) tricyclo[3.3.1.0(3,7)]non-3(7)-ene 6 and cubene 7 were calculated to be 172.5 and 187.4, respectively, at the GIAO-CCSD(T)/tzp/dz//MP2/cc-pVTZ level. In a related study, the relative energies of the various conformers of ethylene were computed and were found to correlate extremely well with the (13)C NMR chemical shifts, reflecting the linear dependence of the (13)C NMR chemical shifts on the internal strain of the molecules.     

Protonated tert-pentyl dication (C5H122+, isopentane dication)

Structures of the tert-pentyl cation (C(5)H(11)(+)) and its protonated dication (C(5)H(12)(2+), isopentane dication) were studied using ab initio methods at the MP2/cc-pVTZ level. Both C-C and C-H hyperconjugatively stabilized structures 1 and 2 , respectively, were found to be minima on the potential energy surface (PES) of the tert-pentyl cation. Structure 1 was computed to be about as stable as structure 2 (slightly more stable by 0.5 kcal mol(-1)). Inter-conversion between 1 and 2 through transition state 3 has a kinetic barrier of only 1.5 kcal mol(-1). The C-H protonated form (H(3)C)(2)C(+)CH(2)CH(4)(+)4 was found to be the global minimum for the protonated tert-pentyl dication. Charges and (13)C NMR chemical shifts of the dication 4 were calculated and compared to those of monocation 1 to study the effect of the additional charge in the dication.     

Cyclopentyl,cyclohexyl, and cycloheptyl cations: computational studies of the structures,stability, <Superscript>13</Superscript>C NMR chemical shifts,and possible rearrangement pathways

Various structures of the cyclopentyl cation were computed at the MP2/cc-pVTZ and CCSD (T)/cc-pVTZ levels. Energetically, the classical cyclopentyl cation 1 and the 1,2-hydrogen-bridged structure 3 were found to be almost identical. The structures and energies of the cyclohexyl and cycloheptyl cations were also calculated at the MP2/cc-pVTZ level. The σ-delocalized nonclassical ion 16 was found to be the lowest energy structure for the cycloheptyl cation. The ¹³C NMR chemical shifts of the ions were also computed and compared with the experimental results.     

Ab initio study of the structures and stabilities of the dimer of ethyl cation, (C(2)H(+)(5))(2) and related C(4)H(10)(2+) isomers.

Ab initio calculations at the MP4(SDTQ)/6-311G//MP2/6-31G level were performed to study the structures and stabilities of the dimer of ethyl cation, (C(2)H(+)(5))(2), and related C(4)H(10)(2+) isomers. Two doubly hydrogen bridged diborane type trans 1 and cis 2 isomers were located as minima. The trans isomer was found to be more favorable than cis isomer by only 0.6 kcal/mol. Several other minima for C(4)H(10)(2+) were also located. However, the global energy minimum corresponds to C-H (C(4) position) protonated 2-butyl cation 10. Structure 10 was computed to be substantially more stable than 1 by 31.7 kcal/mol. The structure 10 was found to be lower in energy than 2-butyl cation 13 by 34.4 kcal/mol.     

IR spectra of C2H5(+)-N2 isomers: evidence for dative chemical bonding in the isolated ethanediazonium ion

The potential energy surface (PES) of C(2)H(5)(+)-N(2) is characterized in detail by infrared photodissociation (IRPD) spectroscopy of mass-selected ions in a quadrupole tandem mass spectrometer and ab initio calculations at the MP2/6-311G(2df,2pd) level. The PES features three nonequivalent minima. Two local minima, 1-N(2)(H) and 1-N(2)(C), are adduct complexes with binding energies of D(0) = 18 and 12 kJ/mol, in which the N(2) ligand is weakly bonded by electrostatic forces to either the acidic proton or the electrophilic carbon atom of the nonclassical C(2)H(5)(+) ion (1), respectively. The global minimum 3 is the ethanediazonium ion, featuring a weak dative bond of D(0) = 38 kJ/mol. This interaction strength is sufficient to switch the C(2)H(5)(+) structure from nonclassical to classical. The 1-N(2)(C) isomer corresponds to the entrance channel complex for addition of N(2) to 1 yielding the product 3. This reaction involves a small barrier of 7 kJ/mol as a result of the rearrangement of the C(2)H(5)(+) ion. The partly rotationally resolved IRPD spectrum of C(2)H(5)(+)-N(2) recorded in the C-H stretch range is dominated by four bands assigned to 3 and one weak transition attributed to 1-N(2)(H). The abundance ratio of 1-N(2)(H) and 3 estimated from the IRPD spectrum as ～1% is consistent with the calculated free energy difference of 12 kJ/mol. As the ethanediazonium ion escaped previous mass spectrometric detection, the currently accepted value for the ethyl cation affinity of N(2) is revised from -ΔH(0) = 15.5 ± 1.5 to ～42 kJ/mol. The first experimental identification and characterization of 3 provides a sensitive probe of the electrophilic character and fluxionality of the ethyl cation. Comparison of 3 with related alkanediazonium ions reveals the drastic effect of the size of the alkyl chain on their chemical reactivity, which is relevant in the context of hydrocarbon plasma chemistry of planetary atmospheres and the interstellar medium, as well as alkylation reactions of (bio)organic molecules (e.g., carcinogenesis and mutagenesis of DNA material).     

Computational studies of 13C NMR chemical shifts of saccharides

The (13)C NMR chemical shifts for alpha-D-lyxofuranose, alpha-D-lyxopyranose (1)C(4), alpha-D-lyxopyranose (4)C(1), alpha-D-glucopyranose (4)C(1), and alpha-D-glucofuranose have been studied at ab initio and density-functional theory levels using TZVP quality basis set. The methods were tested by calculating the nuclear magnetic shieldings for tetramethylsilane (TMS) at different levels of theory using large basis sets. Test calculations on the monosaccharides showed B3LYP(TZVP) and BP86(TZVP) to be cost-efficient levels of theory for calculation of NMR chemical shifts of carbohydrates. The accuracy of the molecular structures and chemical shifts calculated at the B3LYP(TZVP) level is comparable to those obtained at the MP2(TZVP) level. Solvent effects were considered by surrounding the saccharides by water molecules and also by employing a continuum solvent model. None of the applied methods to consider solvent effects was successful. The B3LYP(TZVP) and MP2(TZVP)(13)C NMR chemical shift calculations yielded without solvent and rovibrational corrections an average deviation of 5.4 ppm and 5.0 ppm between calculated and measured shifts. A closer agreement between calculated and measured chemical shifts can be obtained by using a reference compound that is structurally reminiscent of saccharides such as neat methanol. An accurate shielding reference for carbohydrates can be constructed by adding an empirical constant shift to the calculated chemical shifts, deduced from comparisons of B3LYP(TZVP) or BP86(TZVP) and measured chemical shifts of monosaccharides. The systematic deviation of about 3 ppm for O(1)H chemical shifts can be designed to hydrogen bonding, whereas solvent effects on the (1)H NMR chemical shifts of C(1)H were found to be small. At the B3LYP(TZVP) level, the barrier for the torsional motion of the hydroxyl group at C(6) in alpha-D-glucofuranose was calculated to 7.5 kcal mol(-1). The torsional displacement was found to introduce large changes of up to 10 ppm to the (13)C NMR chemical shifts yielding uncertainties of about +/-2 ppm in the chemical shifts.     

13C NMR/DFT/GIAO studies of phenylene bis(1,3-dioxolanium) dications and 2,4,6-triphenylene tris(1,3-dioxolanium) trication

The o-, m-, and p-phenylene bis(1,3-dioxolanium) dications (4-6) and 2,4,6-triphenylene tris(1,3-dioxolanium) trication (7) have been prepared by the ionization of the corresponding 2-methoxyethyl benzoates in FSO(3)H or CF(3)SO(3)H at 40 and 60 degrees C, respectively. The charge delocalization in these carbocations was probed by (13)C NMR chemical shifts and substantiated by GIAO/DFT calculations. Relatively less charge is delocalized into the aromatic ring of the carbotrication 7. The rotational barrier around the C(+)-Ar bond for carbodications 4 and 5 was also estimated to be 8-10 kcal/mol.     

A theoretical (DFT, GIAO-NMR, NICS) study of the carbocations and oxidation dications from azulenes, homoazulene, benzazulenes, benzohomoazulenes, and the isomeric azulenoazulenes

Protonation of parent azulene (1), homoazulene (8), representative isomeric benzazulenes (9, 9A, and 9B), and benzohomoazulenes (10, 10A, and 10B) as well as the mono- and diprotonation of isomeric azulenoazulenes (11-16) were studied by DFT at the B3LYP/6-31G(d) level. The most likely carbocations were identified based on relative protonation energies. For comparison, complete experimental 13C NMR data were obtained for parent azulenium ion 1H+ and guaiazulenium ion 2H+ in TFA. The oxidation dications derived from benzazulenes (9, 9A, and 9B), benzohomoazulenes (10, 10A, and 10B) and azulenoazulenes (11, 16) were also investigated. For azulenoazulene dications the singlet and triplet states are both minima, but except for 11(2+) and 13(2+), the triplet states are higher in energy. Structural/geometrical changes in the carbocations were examined. GIAO-NMR, NPA charges (and changes in charges), and NICS (and delta NICS) were employed to compute the NMR chemical shifts (delta delta 13C values) in order to derive charge delocalization maps and to gauge relative aromaticitylantiaromaticity in the energetically most favored carbocations and oxidation dications. The emerging trends are discussed and compared. Creation of tropylium or homotropylium entities in the carbocations (monoprotonated) and carbodications (diprotonated) is identified as an important driving force governing the protonation outcomes. Consideration of the AM1-derived delta delta Hf values (and the DFT-derived delta delta G values) suggests that the two-electron oxidation of azulenoazulenes should be experimentally feasible. Given their antiaromatic (paratropic) character, azulenoazulene dications represent interesting targets for NMR study. GIAO-derived charge delocalization mapping allows the most likely sites for nucleophilic attack on the dications to be identified.     

Tri- and tetraprotonated ethane (C2H9(3+) and C2H10(4+)) containing five- and six-coordinate carbons

Triprotonated ethane (C(2)H(9)(3+)) 4 and tetraprotonated ethane (C(2)H(10)(4+)) 6 were found by ab initio MP2/cc-pVTZ calculations as viable energy minima. Their structure has three and four two-electron three-center (2e-3c) bonds, respectively. In contrast, calculations showed no minimum-energy structure on the potential energy surface of pentaprotonated ethane (C(2)H(11)(5+)). Charge-charge repulsion may approach its limit in this case. Sufficient stabilization of polycations by Schmidbaur-type auration with (C(6)H(5))(3)PAu, an isolobal analogue of H(+), should be possible for the preparation of the polyaurate derivatives of poly protonated ethane allowing their structural study.     

A study of substituent effect on 1H and 13C NMR spectra of mono,di and poly substituted carbazoles

¹H and ¹³C NMR spectra of several substituted carbazoles (Series 1, 2, 3, 4 and 5) were measured. Substituent chemical shifts (SCS's) and Lynch correlations of ¹H and ¹³C nuclei were calculated and the substituent effect on the NMR phenomena was determined. Atomic charge densities for carbazoles of Series 1, 2, 3, 4 and 5 were calculated by using the semi empirical PM3 method. These values also show a linear correlation with the ¹³C chemical shifts.     

C7H12(2+): a prototype hexacoordinate carbonium ion

C(7)H(12)(2+) (1), the prototype hexacoordinate carbonium dication was found to be a viable minimum at the MP2/6-31G** and MP2/cc-pVTZ levels. Structure 1 is a propeller shaped molecule resembling a complex involving a C(2+) with three ethylene molecules resulting in the formation of three two-electron, three-center (2e-3c) bonds. Isomeric structure 2 was found to be 21.8 kcal/mol more stable than structure 1. However, conversion of 1 into 2 through transition structure 3 has a barrier of 5.7 kcal/mol. Related structures 4, 5, and 8 were also located as minima for C(7)H(12)(2+). The isoelectronic boron analogue BC(6)H(12)(+) (10) was also computed to be a minimum at the same level of calculations.     

Visualization of homoaromaticity in cations, neutral molecules and anions by spatial magnetic properties (through space NMR shieldings)—an H/C NMR chemical shift study

Prototypes for homoaromaticity in cations, neutral molecules, and anions are theoretically studied at the MP2 level of theory. For the global minimum structures on the potential energy surface both ¹H/¹³C chemical shifts and spatial magnetic properties as through space NMR shieldings (TSNMRS) were calculated by the GIAO perturbation method. The TSNMRS are visualized as iso-chemical-shielding surfaces (ICSS) of different sign and size. Coincident experimental and computed ¹H/¹³C chemical shifts afforded the possibility to decide from the TSNMRSs at hand on both the existence and the size of homoaromaticity in the molecules studied.     

Complexes of guanidinium ion (NH2)3C+ with super Lewis acidic XH4+(X = B and Al): comparison with XH3 complexes and protonated and methylated guanidinium dications

Structures of the complexes (1 and 8) of the guanidinium ion (H(2)N)(3)C(+) with super Lewis acidic BH(4)(+) and AlH(4)(+) were calculated using the DFT method at the B3LYP/6-311+G** level. (13)C NMR chemical shifts were also calculated by the GIAO-MP2 method. Each of the dicationic complexes contains a hypercoordinate boron or aluminum atom with a two-electron three-center (2e-3c) bond. Guanidinium ion was found to form a strong complex with BH(4)(+) but a relatively weak one with AlH(4)(+). On the other hand, complexations of guanidinium ion with neutral BH(3) and AlH(3) lead only to very weak complexes (5 and 9). The structures of mono- and dicationic complexes were compared with the structures of protonated and methylated guanidinium dications.     

Carbocations (M + H)(+) and oxidation dications (M(2+)) from benzo[a]pyrene and its nonalternant isomers azulenophenalenes: a theoretical (DFT, GIAO, NICS) study

The arenium ions of protonation and the two-electron oxidation dications derived from benzo[a]pyrene (BaP) 1 and three of its nonalternant isomers namely azuleno[5,6,7-cd]phenalene 2 (a strong carcinogen reported to be as potent as BaP) azuleno[1,2,3-cd]phenalene 3 (a strong mutagen/weak carcinogen), and azuleno[4,5,6-cd]phenalene 4 (a weak mutagen) were studied by DFT at the B3LYP/6-31G(d) level. The most favored sites for electrophilic attack were identified on the basis of relative protonation energies in the arenium ions. Computed NMR chemical shifts (GIAO NMR), the NPA-derived charges (and changes in charges), as well as NICS (and DeltaNICS) were employed to derive charge delocalization maps and to gauge relative aromaticity/antiaromaticity in the resulting carbocations and oxidation dications. Quantitative correlations between the experimental (superacid) (13)C data and GIAO chemical shifts, and between computed changes in charges and GIAO Deltadelta (13)C values were explored for benzo[a]pyrenium ion (1cH(+)) and its singlet oxidation dication (1(2+)) as representative cases. For the studied PAHs (1-4), formation of singlet dications were computed to be strongly favored except in 4 for which the triplet lies 5 kcal/mol lower than singlet. Relative carbocation stability data and the derived charge delocalization patterns are assessed in light of the available chemical and toxicological data on these compounds. The present study is the first of its kind to examine the carbocations and oxidation dications derived from biologically active nonalternant analogues of BaP for which no stable ion data are available. It also validates and extends the experimental data for BaP carbocation and oxidation dication and provides a means to gauge the success of GIAO NMR in predicting NMR data for PAH-arenium ions.     

Preparation and NMR study of silylated carboxonium ions

A series of silylated carboxonium ions, 2a-6a, were prepared as long-lived species by treating triethylsilane and triphenylmethyl tetrakis(pentafluorophenyl)borate (Ph3C(+)B(C6F5)4-) with ketones, enones, carbonates, amides, and urea in CD2Cl2 solution. They were characterized by 13C and 29Si NMR spectroscopy at -78 degrees C. The NMR study indicates that the silylated carbonyl compounds are resonance hybrids of oxocarbenium and carboxonium ions, while the latter are the major contributors to the overall structures. The structure and 13C and 29Si NMR chemical shifts of the model trimethylsilylated carboxonium ions were also calculated by density functional theory/IGLO methods. The calculated results agree well with the experimental data.     

Experimental (FT-IR, FT-Raman, H, C NMR) and theoretical study of alkali metal 2-aminonicotinates

The influence of lithium, sodium, potassium, rubidium and cesium on the electronic system of 2-aminonicotinic acid (2-ANA) was studied by the methods of molecular spectroscopy. The vibrational (FT-IR, FT-Raman) and NMR (¹H and ¹³C) spectra of 2-aminonicotinic acid and its alkali metal salts were recorded. Characteristic shifts and changes in intensities of bands along the metal series were observed. The changes of chemical shifts of protons (¹H NMR) and carbons (¹³C NMR) in the series of studied alkali metal 2-aminonicotinates (2-AN) were observed too.Optimized geometrical structures of the studied compounds were calculated by the B3LYP method using the 6-311++G** basis set. Aromaticity indices, atomic charges, dipole moments and energies were also calculated. The theoretical chemical shifts in ¹H and ¹³C NMR spectra and theoretical wavenumbers and intensities of IR and Raman spectra were determined. The calculated parameters were compared to the experimental characteristics of the studied compounds.