Search

An anionic three-way switch A bipyridyl bisurea-based anion receptor that is highly selective for dihydrogen phosphate demonstrates spectroscopically distinct anion bound conformations toward halides and select oxoanions. 1H NMR studies show the differing anion induced conformations are reversible allowing this system to function as a three-way molecular switch.

Antiaromatic dications and dianions are available through oxidation or reduction of unsaturated precursors as well as through ionization of diols. Their experimental accessibility allows them to be used to evaluate the effectiveness of theoretical treatments of antiaromaticity such as the nucleus-independent chemical shift (NICS). Because measures of antiaromaticity are generally larger in magnitude than corresponding measures of aromaticity, they demonstrate relationships between different measures of aromaticity–antiaromaticity, such as those based on magnetic and energetic properties, more effectively than do aromatic measures. Antiaromatic species are more sensitive probes of delocalization than are aromatic species, giving the possibility of observation of subtle effects. Antiaromatic species possess a small highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO-LUMO) gap, giving rise to species with diradical character.

The antiaromaticity of a series of dianions of p-substituted benzylidene dibenzo[a,d]cycloheptenes was examined through calculated measures of antiaromaticity. The nucleus-independent chemical shifts (NICS) and magnetic susceptibility exaltation both showed substantial antiaromatic character in the benzannulated tropylium anion. When the antiaromaticity was normalized for the area of the ring, these tropylium anions were shown to be among the most antiaromatic anions in the chemical literature. Attempts to make the dianion through reduction with lithium or potassium gave the tetraanion as the only species observable in the (1)H NMR spectrum. Quench of the reaction mixture with trimethylsilyl chloride or D(2)O confirmed the presence of the tetraanion, but only as a small portion of the reaction mixture, with the major product being unreacted starting material. The failure to observe starting material was attributed to similarities in the structures of the starting material and anion radical (first reduction), allowing rapid electron transfer between them. The inability to see the dianion (second reduction) could be the result of the very small HOMO-LUMO gap anticipated for highly antiaromatic species, which would allow access to diradical species. The magnitude of the HOMO-LUMO gap was determined by the difference between the HOMO and LUMO energies from geometry optimization and the lowest energy transition from TD-DFT calculations. The HOMO-LUMO gap for the benzylidene dibenzocycloheptatriene dianions was shown to be much smaller than the HOMO-LUMO gap of species for which (1)H NMR spectra had been observed.

Students at two-year colleges and small four-year colleges have often obtained their exposure to NMR spectroscopy through “canned” spectra because the cost of an NMR spectrometer, particularly a high-field spectrometer, is prohibitive in these environments. This article describes the design of a NMR site at Trinity University in which spectral data from student samples from community colleges and several four-year colleges are obtained on a 300 MHz NMR spectrometer. The unprocessed free induction decay (FID) is distributed to the students via the Internet and then processed by the students at their local institutions. The success of the NMR site at one two-year college, Palo Alto College, is described along with details of the experiments chosen for analysis. Two other two-year colleges, San Antonio College and Northwest Vista College, provide additional information about the effectiveness of the site.

Reduction of dixanthylidene with potassium or lithium resulted in formation of the antiaromatic dianion in high yield. Attempts to form the dianion by excited-state deprotonation of dixanthene with n-butyllithium/TMEDA resulted in formation of the tetraanion from deprotonation ortho to the oxygen. Orientation of the sp(3) hydrogens presumably allows preferential deprotonation of the xanthene rings.

Dications of 9-(3-phenyl-1H-inden-1-ylidene)-5H-dibenzo[a,d]cycloheptene, 5(2+), were prepared by oxidation with SbF(5) in SO(2)ClF, and their magnetic behavior was compared to dications of 9-(3-phenyl-1H-inden-1-ylidene)-9H-fluorene, 2(2+). The good correlation between the experimental (1)H NMR shifts for the dications that were oxidized cleanly and the chemical shifts calculated by the GAIO method supported the use of the nucleus independent chemical shifts, NICS, to evaluate the antiaromaticity of the indenyl systems of 2(2+)/5(2+) and their unsubstituted parent compounds, 6(2+) and 7(2+), as well as the antiaromaticity of the fluorenyl system of 2(2+)/7(2+) and the aromaticity of the dibenzotropylium system of 5(2+)/6(2+). Antiaromaticity was shown to be directly related to the amount of charge in the antiaromatic systems, with the antiaromatic systems more responsive to changes in the calculated NBO charge than the aromatic systems. The antiaromaticity was also shown to be directly related to the amount of delocalization in the ring system. The aromaticity of the dibenzotropylium system was much less responsive to changes in the amount of charge in the tropylium system, because the aromatic system was much more completely delocalized. Thus, antiaromatic species are more sensitive probes of delocalization than aromatic ones.

Derivatives of the dication of tetrabenzo[5.5]fulvalene were prepared with phenyl and ethynyl spacers through ionization of the appropriate bis-methylethers. The antiaromaticity shown by the parent dication was demonstrated for these dications with spacers, although it was attenuated by the presence of the spacer. It was substantially greater than that of fluorenyl monocations with similar substituents. Antiaromaticity was evaluated through comparison of (1)H NMR shifts with those of acyclic analogues, through nucleus independent chemical shifts, and through magnetic susceptibility exaltation. Although the fluorenyl systems are separated by spacers, the antiaromaticity of one system is affected by the other remote fluorenyl system. An explanation for this interaction may lie in the ability of a remote cationic substituent to attenuate delocalization in the spacer. The use of spacers is designed to prevent side reactions in less stable antiaromatic dications, allowing exploration of a number of species that have previously been inaccessible.

When the nucleus independent chemical shifts, NICS(1)zz, for a set of aromatic and antiaromatic hydrocarbons are summed, they show an excellent linear relationship with the magnetic susceptibility exaltation, Lambda, for neutral, cationic, and monoanionic species. Aromatic and antiaromatic dianions show a similar relationship but with a different slope. However, when both Lambda and the summation of NICS(1)zz are divided by the area of the ring squared, the vast majority of the aromatic and antiaromatic species fall on the same line, indicating that both NICS and Lambda are affected by the size of the ring. The species that deviate slightly from the line are a few of the anionic compounds, which may be a result of the difficulties of calculating magnetic properties of anions. This is the first report of the relationship of NICS to ring area. In addition, the excellent correlation between Lambda and the summation of NICS(1)zz demonstrates that summation of NICS(1)zz values for individual ring systems of polycyclic ring systems to give a measure of the aromaticity of the entire system is justified. By extension, the excellent correlation also serves to demonstrate that the NICS(1)zz values for individual ring systems are reliable measures of local aromaticity/antiaromaticity. Finally, the excellent correlation between experimental shifts and the 13C NMR shifts calculated with density functional theory, B3LYP/6-311+g(d,p), serves as an indirect validation of the accuracy of the NICS(1)zz calculated by the same method and basis set.

[reaction: see text] Electrochemical oxidation of meta-substituted diphenylmethylidenefluorenes (3a-g) results in the formation of fluorenylidene dications that are shown to be antiaromatic through calculation of the nucleus independent chemical shift (NICS) for the 5- and 6-membered rings of the fluorenyl system. There is a strong linear correlation between the redox potential for the dication and both the calculated NICS and sigma(m). Redox potentials for formation of dications of analogously substituted tetraphenylethylenes shows that, with the exception of the p-methyl derivative, the redox potentials for these dications are less positive than for formation of the dications of 3a-g and for dications of p-substituted diphenylmethylidenefluorenes, 2a-g. The greater instability of dications of 2a-g and 3a-g compared to the reference system implies their antiaromaticity, which is supported by the positive NICS values. The redox potentials for formation of the dications of meta-substituted diphenylmethylidenes (3a-g) are more positive than for the formation of dications of para-substituted diphenylmethylidenes (2a-g), indicating their greater thermodynamic instability. The NICS values for dications of 3a-g are more antiaromatic than for dications of 2a-g, which is consistent with their greater instability of the dications of 3a-g. Although the substituted diphenylmethyl systems are not able to interact with the fluorenyl system through resonance because of their geometry, they are able to moderate the antiaromaticity of the fluorenyl cationic system. Two models have been suggested for this interaction, sigma to p donation and the ability of the charge on the substituted ring system to affect delocalization. Examination of bond lengths shows very limited variation, which argues against sigma to p donation in these systems. A strong correlation between NICS and sigma constants suggests that factors that affect the magnitude of the charge on the benzylic (alpha) carbon of the diphenylmethyl cation affect the antiaromaticity of the fluorenyl cation. Calculated atomic charges on carbons 1-8 and 10-13 show an increase in positive charge, and therefore greater delocalization of charge in the fluorenyl system, with increasing electronegativity of the substituent. The change in the amount of positive charge correlated strongly with NICS, supporting the model in which the amount of delocalization of charge is related to the antiaromaticity of the species. Thus, both aromatic and antiaromatic species are characterized by extensive delocalization of electron density.

Aromatic stabilization energy (ASE) calculations for the fluorenyl cation show substantial destabilization in comparison to suitable reference systems (16.3 +/- 1.6 kcal/mol), supporting its categorization as an antiaromatic species. The choice of appropriate reference systems is exacting for cationic systems because of the need to match strain energies, convolved with allylic-type resonance terms and other potential structural effects that stabilize charge. Several homodesmotic ASE reaction systems are examined to demonstrate the role played by these factors in the calculation of an ASE value for the fluorenyl cation. The magnitudes of the derived ASE are quite similar for four very different determinative, homodesmotic reaction systems, giving strong support to the inherent accuracy of the final derived ASE value. The results of nucleus independent chemical shift calculations for the components of each one of the ASE reactions add additional weight to this conclusion.

The NICS values of the dication of tetrabenzo[5.5]fulvalene (1) show substantial antiaromaticity. Substituted fluorenyl cations possess antiaromatic five-membered rings but the calculated antiaromaticity (NICS) of the six-membered rings depends on the calculational level. Through calculation of magnetic susceptibility exaltation (Λ), 1 is antiaromatic while fluorenyl cations are not. The paratropic shift seen in the 1H NMR spectra of 1 and the substituted fluorenyl cations is linearly related to NICS (six-membered ring) and to Λ. NICS values suggest that electron delocalization in the fluorenyl cations occurs to maintain the aromaticity of the benzene subunits and to localize the positive charge in the five-membered ring. In contrast, electron delocalization in 1 results in delocalization of a positive charge throughout each fluorenyl system.

Oxidation of para-substituted diphenylmethylidenefluorenes results in the formation of antiaromatic fluorenyl cations, as evidenced by the paratropic shift in the 1H NMR spectrum. The magnitude of ...

Oxidation of 2,7-disubstituted tetrabenzo[5.5]fulvalene derivatives 1a−d resulted in the formation of dications which are fluorenyl cations linked by a single bond. These fluorenyl cations exhibit significant paratropicity in the 1H NMR spectrum, which is attributed to an antiaromatic ring current. Interaction of the perpendicular ring systems is evident in the upfield shift of carbons a and a‘, presumably due to σ−p donation. The lack of variation in the upfield shift of carbons a and a‘, compared to previously reported systems, is attributed to the similarities of the geometries of 1a−d2+. Substitution at a remote site affects the antiaromaticity of the unsubstituted fluorenyl cation, but the nature of the effect of the substituent is not understood. Direct substitution on the fluorenyl cation by substituents which increase electron density, either through inductive π polarization or through resonance, cause a paratropic shift in the probe proton.

Synthesis of diindeno[1,2,3,4-defg: 1′,2′,3′,4′-mnop]chrysene (1), a portion of the C60 surface, was attempted through oxidative cyclization of tetrabenzo[5.5]fulvalene (2), dibenzo[gp]chrysene (3), and diphenylmethylidenefluorene (4) by SbF5/SO2CIF. Compounds 2 and 3 were oxidized to dications which then underwent a single cyclization to give precursors to 1. Compound 4 underwent two oxidative cyclizations to give a precursor to 1. AM1 calculations of the possible products from cyclization were consistent with preferential formation of the cyclized product with the lower ΔHf. Oxidative cyclization may offer a one-pot synthetic alternative for the preparation of unusual polycyclic aromatic hydrocarbons.

Oxidation of tetrabenzofulvalene derivatives 1−5 containing fluorenylidene and 5−7-membered rings resulted in the formation of dications which are effectively fluorenyl cations with perpendicular cyclic substituents. The observed paratropicity of the fluorenyl cation in these systems is attributed to an antiaromatic ring current, after evaluation of the effects of geometry, charge density, and polarity of the medium, and is dependent on the geometry and electronic character of the cyclic substituent. A linear relationship between the 1H shifts of appropriate protons of the fluorenyl system and 13C shift of carbon a suggests that the effect of the substituent is transmitted through cross-hyperconjugation.

Antiaromatic species tend to be among the least well-studied and understood group in organic chemistry, primarily because their anticipated instability dissuaded most chemists from exploring their behavior. A research project that was intended to form three-dimensional aromatic dications resulted instead in a set of antiaromatic dications from oxidation of fluorenylidene derivatives. These dications can be modified in ways that help illuminate factors that affect both antiaromaticity and aromaticity. The characterization of these species, as well as antiaromatic dianions, through magnetic, energetic, and structural properties is described along with the relationships between these properties. Because this work represents contributions almost exclusively from undergraduate researchers, and because many readers of the Journal may not have a great deal of experience in this environment, I also include some thoughts about the opportunities and challenges of undergraduate research.

Antiaromatic species tend to be among the least well-studied and understood group in organic chemistry, primarily because their anticipated instability dissuaded most chemists from exploring their behavior. A research project that was intended to form three-dimensional aromatic dications resulted instead in a set of antiaromatic dications from oxidation of fluorenylidene derivatives. These dications can be modified in ways that help illuminate factors that affect both antiaromaticity and aromaticity. The characterization of these species, as well as antiaromatic dianions, through magnetic, energetic, and structural properties is described along with the relationships between these properties. Because this work represents contributions almost exclusively from undergraduate researchers, and because many readers of the Journal may not have a great deal of experience in this environment, I also include some thoughts about the opportunities and challenges of undergraduate research.

A new method for the generation of dicationic species via ionization of diols is described. The method makes use of milder reagents at room temperatures, an advantage over use of Magic Acid at −78 °C. A series of mono- and dications were synthesized successfully, including previously unattainable species.

Oxidation of m- and p-substituted benzylidene fluorenes to antiaromatic dications was attempted by electrochemical and chemical means. Electrochemical oxidation to dications was successful for benzylidene fluorenes with p-methoxy, p-methyl, p-fluoro, and unsubstituted phenyl rings in the 3-position; attempts to oxidize the m-substituted derivatives via electrochemistry were unsuccessful. Chemical oxidation with SbF(5)/SO(2)ClF gave the dication of 9-[(4-methoxyphenyl)methylene]-9H-fluorene cleanly; oxidation of all other substituted benzylidene fluorenes resulted in mixtures of products. The excellent linear relationship between the chemical shifts calculated by the GIAO method and the experimental shifts for the p-methoxy-substituted benzylidene fluorene dication suggests that the calculations satisfactorily reflect the magnetic properties of this dication and potentially those of the other dications studied. The redox potentials from electrochemical oxidation, a measure of the stability of the dications, showed a good linear relationship with another measure of stability, the calculated difference in energy between each dication and its neutral precursor. The dications of benzylidene fluorenes were less stable than the dications of diphenylmethylidene fluorenes; within each type of compound, dications with p-substituted phenyl rings were more stable than dications with m-substituted phenyl rings and dications with phenyl rings substituted with electron-donating groups were more stable than dications with phenyl rings substituted with electron-withdrawing groups. The antiaromaticity of the fluorenyl system was assessed through the nucleus-independent chemical shift (NICS) that was also calculated by the GIAO method. The plot of the NICS values per square area versus the calculated energy difference for the dications showed a moderate degree of linearity; the plot of NICS values per square area versus the oxidation potentials was less linear. Thus, a suggestive, but not conclusive, relationship between magnetic and energetic measures of antiaromaticity was observed.

Oxidation of 2,7-disubstituted tetrabenzo[5.5]fulvalene derivatives 1a−d resulted in the formation of dications which are fluorenyl cations linked by a single bond. These fluorenyl cations exhibit significant paratropicity in the 1H NMR spectrum, which is attributed to an antiaromatic ring current. Interaction of the perpendicular ring systems is evident in the upfield shift of carbons a and a‘, presumably due to σ−p donation. The lack of variation in the upfield shift of carbons a and a‘, compared to previously reported systems, is attributed to the similarities of the geometries of 1a−d2+. Substitution at a remote site affects the antiaromaticity of the unsubstituted fluorenyl cation, but the nature of the effect of the substituent is not understood. Direct substitution on the fluorenyl cation by substituents which increase electron density, either through inductive π polarization or through resonance, cause a paratropic shift in the probe proton.

Oxidation of para-substituted diphenylmethylidenefluorenes results in the formation of antiaromatic fluorenyl cations, as evidenced by the paratropic shift in the 1H NMR spectrum. The magnitude of ...

The ability of unsaturated hydrocarbons to be oxidized to dications by SbF 5 was examined through AM1, MNDO, and MINDO/3 calculations on 53 compounds whose reactions under these conditions were reported in the literature. A positive correlation existed for first ionization potentials calculated by AM1 for substituted and unsubstituted polycyclic aromatic hydrocarbons, with oxidation to dications occurring when the ionization potential was ≤8.3 eV. The correlation was poor for nonbenzenoid precusors

Dimetalation of 1,4-bis(methylene)cyclohexane results in formation of the p-xylene dianion. In contrast, 2-methyl-1,5-hexadiene, an acyclic analogue of 1,4-bis(methylene)cyclohexane, undergoes isomerization

The effect of spacers on the antiaromaticity of bisfluorenyl dications was examined. For systems such as 4a-c in which extensive delocalization of the positive charge through the pi-system is not possible, a relationship based primarily on localized hyperconjugation as well as the distance between the fluorenyl systems and their antiaromaticity was demonstrated. Dications 2a-c and 3a-c also show the relationship, but it is further modulated by delocalization through the pi-system. Calculated assessment of antiaromaticity was supported by experimental shifts for 2a,b and 3a-c.

The dianion, 5(2-), and dication, 5(2+), of tetrabenzo[5.7]fulvalene represent an aromaticity/antiaromaticity continuum in which the fluorenyl system changes from aromatic in 5(2-) to antiaromatic in 5(2+). Conversely, the antiaromatic dibenzotropylium system of 5(2-) becomes an aromatic system in 5(2+), allowing an examination of aromaticity/antiaromaticity within the same carbon framework. Dianion 5(2-) was prepared and characterized by (1)H NMR spectroscopy. The fluorenyl system of 5(2-) showed the downfield shifts expected for an aromatic system, while the dibenzotropylium system showed the paratropic shifts expected for an antiaromatic system. The conclusions from (1)H NMR spectroscopy were supported by NICS(1) zz calculations for each system. Comparison of the (1)H NMR spectrum and NICS(1) zz of 5(2-) with those of 5(2+) supported the assignments of aromaticity/antiaromaticity for each system. Aromaticity/antiaromaticity were further examined through comparison of the degree of bond length alternation, which showed that the bond length alternation was slightly greater for the antiaromatic ring systems than for the aromatic systems. However, when structures of 5(2-) and 5(2+) with no bond length alternation were examined, there was a dramatic increase in the degree of antiaromaticity for the antiaromatic ring systems as evaluated through NICS. This result suggests that a decrease in bond length alternation results in an increase in antiaromaticity as well as an increase in aromaticity. The magnitude of the antiaromaticity of the fluorenyl system in 5(2+) was greater than the magnitude of the aromaticity in the fluorenyl system of 5(2-), with similar effects shown by the analogous tropylium systems. This is consistent with the behavior of the antiaromatic dication of tetrabenzo[5.5]fulvalene, compared to that of its aromatic dianion, and also with the behavior of the cyclopentadienyl cation/anion and tropylium cation/anion.

Fluorenylidene dications, such as the dications of p- and m-substituted diphenylmethylidenefluorenes, show appreciable antiaromaticity. Evidence of antiaromaticity is demonstrated through 'H NMR shifts, nucleus independent chemical shifts (NICS), magnetic susceptibility exaltation, A, and (anti)aromatic (de)stabilization energies, ASE. Extension of the research to indenylidenefluorene dications shows that, contrary to expectation, the indenyl cation in these dications is less antiaromatic than the fluorenyl cation. The magnitude of the antiaromaticity is evaluated through comparison to the aromaticity of related dianions and reveals that the fluorenylidene dications are more antiaromatic than the fluorenylidene dianions are aromatic.

Dications of p-substituted 3-phenylindenylidenefluorenes were prepared to examine the response of the resulting indenyl and fluorenyl cationic systems to magnetic measures of antiaromaticity. All measures, (1)H NMR shifts, nucleus independent chemical shifts (NICS(1)(zz)), and magnetic susceptibility exaltation, Lambda, supported the antiaromaticity of the dications 3a-f2+. The 1H NMR shifts and NICS(1)(zz) showed that the indenyl ring system was less antiaromatic than the fluorenyl ring system, contrary to the antiaromaticity of indenyl monocations compared to fluorenyl monocations. The presence of a phenyl substituent in the 3-position was able to stabilize the indenylidene cation through resonance, decreasing its antiaromaticity, but even in the absence of the 3-phenyl substituent, the indenyl system of indenylidenefluorene dications was less antiaromatic than the fluorenyl system. The decreased antiaromaticity of the 3-phenylindenylidenefluorene dications over the unsubstituted indenylidenefluorene dication was supported by (anti)aromatic (de)stabilization energy calculations, ASE.

The aromaticity of the dianion (2) and the antiaromaticity of the dication (3) of tetrabenzo[5.5]fulvalene have been evaluated through magnetic criteria, (1)H NMR shifts, nucleus-independent chemical shifts, NICS, and magnetic susceptibility exaltation, Lambda. The sum of the NICS values, using the GIAO (gauge-independent atomic orbital) method, for 2 is -35.2; that of 3 is +38.2, indicating the aromaticity of 2 and the antiaromaticity of 3. Calculation of magnetic susceptibility exaltation using the CSGT (continuous set of gauge transformations) method gives a similar result, with Lambda of -81.8 ppm cgs for 2 and 95.8 ppm cgs for 3. The general validity of these values is supported by excellent agreement between the NMR shifts calculated by the GIAO and CSGT methods with experimental shifts. Comparison of 1H NMR shifts with those of model compounds allows evaluation of the magnitude of the diatropic shift in 2 and paratropic shift in 3 and supports their assignment as aromatic/antiaromatic, respectively. The agreement between calculated and experimental 1H NMR shifts is excellent for 3 in the absence of counterions but much better for 2 when counterions are included. Inclusion of counterions in the evaluation of diatropic shift for 2 gave a smaller shift than in the absence of counterions, suggesting a decreased aromaticity. When counterions were included in the calculation of Lambda, the value was also decreased, suggesting a decreased aromaticity. This observation has important consequences in the use of experimental data for the evaluation of aromaticity, and presumably antiaromaticity, of anions since, in most cases, there will be close interaction with counterions.

The relationship between the calculated energy of the HOMO-LUMO gap, where (epsilonLUMO - epsilonHOMO)/2 is defined as DeltaHL, and of the longest wavelength transition in the UV-visible spectrum, DeltaE, was examined for a series of aromatic and antiaromatic cations and dications. TD-DFT calculations accurately modeled the energies of a series of dications including fluorenylidene dications whose UV-visible spectra are reported, as well as the energies of a series of aromatic and antiaromatic monocations whose spectra were previously reported. There is a linear correlation of the energy of the longest wavelength transition, DeltaEcalc, with DeltaHL. There is no linear relationship between DeltaEcalc and the sum of the NICS values, but there is a linear relationship between DeltaEcalc multiplied by the number of atoms in the conjugated system, DeltaEcalcT, and the sum of the NICS values. There is also an approximate linear relationship between the average 1H NMR shift and the sum of the NICS values. These relationships give further support to the suggestion that the magnetic and energetic criteria of aromaticity and antiaromaticity are related. Furthermore, the data suggest that species that have DeltaEcalcT20 are antiaromatic whereas those with DeltaEcalcT30 are aromatic.

Oxidation of 3,6-disubstituted tetrabenzo[5.5]fulvalenes by SbF(5) results in the formation of dications that behave like two antiaromatic fluorenyl cations connected by a single bond. Both fluorenyl systems exhibit the paratropic shifts and nucleus independent chemical shifts (NICS) characteristic of antiaromatic species. Comparison with analogous 2,7-disubstituted tetrabenzo[5.5]fulvalenes reveals that the antiaromaticity of the substituted ring system can be altered substantially by changes in the placement of the substituents, possibly due to changes in the delocalization of charge in the system. Substituents in the 3,6-position decrease the antiaromaticity because of the increase in the benzylic resonance compared to 2,7-substituents.

The antiaromaticity of fluorenylidene dications 1-5, which contain cyclic cationic substituents, has been examined using magnetic criteria, NICS and magnetic susceptibility, and by structural criteria, HOMA. The magnetic criteria, including proton chemical shifts, strongly support the antiaromaticity of the fluorenyl ring system of these dications. HOMA values are a very insensitive measure of structural effects in polycyclic ring systems because they reflect the inability of the fused ring systems to respond to changes in aromaticity/antiaromaticity. Finally, in these systems, the interaction between the ring systems appears to occur primarily through a type of hyperconjugation, as demonstrated by a decrease in the bond lengths for the bonds connecting the ring systems. In conjunction with the evaluation of magnetic effects, the quality of the calculation of (1)H and (13)C NMR shifts was assessed by comparison with experimental data. The calculation of (13)C NMR shifts was excellent in all methods examined, but the quality of (1)H NMR shifts was substantially poorer in calculations using the IGLO method, basis set DZ or II. The CSGT method gives a superior correlation between experimental and calculated (1)H NMR shifts.

The antiaromaticity of a series of dications of p-substituted diphenylmethylidene fluorenes was explored using three criteria attributed to aromaticity/antiaromaticity. The relative stability of the dications (energetic criterion) was measured via the redox potentials obtained by electrochemical oxidation under very fast sweep rates with microelectrodes. Comparison of redox potentials with those of a model system, p-substituted tetraphenylethylenes, shows relatively small destabilization of the potentially antiaromatic fluorenylidene dication. However, the amount of destabilization is comparable with the limited electrochemical data available for other antiaromatic systems. Nucleus independent chemical shifts (NICS) were calculated for these dications (magnetic criterion) and indicated their antiaromaticity. A good linear relationship between experimental and calculated (B3LYP/6-31G(d)) (1)H and (13)C NMR shifts for the three dications, 3c, 3e, and 3f, for which NMR data has been reported, validated the accuracy of the NICS values. Bond length alternation/elongation (structural criterion) was explored via the harmonic oscillator model of aromaticity (HOMA) using the geometries calculated with density functional theory, but there was insufficient variation to evaluate relative antiaromaticity. In addition, the presence of benzannulation appears to restrict bond length alternation to such an extent that the magnitude of the HOMA index is of little use in evaluating the antiaromaticity of many polycyclic hydrocarbons. Both NICS values and redox potentials for formation of the dication in these systems show a strong linear correlation with sigma(p)(+) values, with the more antiaromatic fluorenylidene dication possessing the more electron-withdrawing substituent. The correlation between NICS values and redox potentials is also good, as might be expected, suggesting a strong relationship between magnetic and energetic characteristics of antiaromaticity. However, magnetic characteristics appear to be a more sensitive probe than energetic characteristics evaluated through redox potentials or structural characteristics evaluated through HOMA calculations.

The antiaromaticity of fluorenyl cations is dependent on the magnitude of the charge of the system. Theoretical assessments of antiaromaticity and charge were supported by experimental NMR chemical shifts. Delocalization was related to antiaromaticity, and evaluated through the standard deviation of the charges on carbons of the fluorenyl systems.

The Chemistry Division of the Council on Undergraduate Research (CUR) provides a mentoring service for prospective, new, and established faculty seeking information on various aspects of initiating and enhancing undergraduate research opportunities. Experienced CUR members provide advice on such issues as getting started in reearch, mentoring undergraduate researchers, obtaining tenure-track positions at PUIs, developing research-supportive curricula, reinvigorating research programs, and establishing collaborations. CUR Chemistry Division members are invited to offer their services as mentors or to inquire about finding a mentor to discuss career questions and options.

Capillary electrophoresis is gradually working its way into the undergraduate laboratory curriculum. Typically, experiments utilizing this technology have been introduced into analytical or instrumental courses. We have introduced an experiment into the organic chemistry sequence, in the synthesis laboratory, that utilizes capillary electrophoresis to evaluate the pKa shift on a series of student-prepared substituted benzoic acids. The pKa shift is examined in reference to the electrophoretic-migration behavior of benzoic acid and is a result of the electron-withdrawing or electron-donating characteristics of the substituent. This strategy allows us to increase the exposure of students to modern methods of separation. We are using repeated exposure to increase students' knowledge of separation strategies and techniques. Through this process students will gain a better, fuller understanding of CE than they would have with a single exposure in the analytical or instrumental laboratory. In our curriculum, th...

As part of its efforts toward enhancing collaborative student-faculty scholarship, CUR has established a series of workshops focused on issues of concern to faculty involved in research with undergraduates. Two of those workshops will be presented in the coming year: "Institutionalizing Undergraduate Research" will be held at the University of Wisconsin-La Crosse at the end of October 1999; and "The Vital Faculty: Issues after Tenure" will be held at the College of William and Mary in mid-February 2000.

This book, part of a four-volume series planned to deal with all aspects of a standard NMR experiment, is almost the exact book I have been hoping to find. My department has acquired, as have hundreds of other undergraduate institutions, high-field NMR instrumentation and the capability of doing extremely sophisticated experiments. However, the training is often a one- or two-day experience in which the material retained by the faculty trained is garbled and filled with holes, not unlike the information our students seem to retain. This text, and the accompanying exercises based on data contained on a CD-ROM, goes a long way to fill in the gaps and clarify misunderstandings about NMR processing. The text uses the Windows-based program WIN-NMR, a Bruker software product, to demonstrate processing 1- and 2-D spectra. The program and NMR data sets are contained on a CD-ROM and require an IBM-compatible 386, 486 or Pentium class PC with a minimum of 4 MB RAM. A hard disk with at least 100 MB spare capacity is recommended. I was initially skeptical about the value of the approach because I am only familiar with Varian software and am a Mac user; but I found the Windows-based program almost as easy to use as a Mac program. More importantly, although the WIN-NMR software uses very different commands from Varian software, the philosophy of the approach was so transparent as to make it reasonably straightforward to take what was learned via WIN-NMR and apply it in a Varian setting. The book begins with a section that describes the various types of NMR experiments, including the phase-sensitive double quantum filtered COSY, TOCSY (total correlation spectroscopy), and ROESY (rotating frame Overhouser effect spectroscopy) experiments. The descriptions are enhanced editorial comments that contrast the various experiments, suggesting the situations in which each is most appropriate. The remainder of the book is a set of exercises using data on per-acetylated I²-D-glucose from a wide variety of NMR experiments. The reader is led through simple commands on already processed FIDs through sophisticated processing of 1- and 2D data sets. The real advantage of the exercises lies in two areas. First, the authors give nuts and bolts suggestions on processing strategies that are far more helpful than I have found in any other reference. Second, the processing and display is done on good data sets. When one is unable to extract information from one's own experiments, it is difficult to determine whether the fault is in the processing or in the data itself. These data are good, so one area of ambiguity is removed. In addition, all data sets contain both the FID and a data set that is already Fourier transformed and phased. Thus you can see what the data should reveal and have a standard against which to compare your own attempts. My only real complaint with the text was the authors' tendency to instruct the reader to explore processing variations until the result looks good. For a novice, the ability to discern good is pretty poor. The authors led me through processing and displaying phase-sensitive data sets like phase-sensitive COSY, as well as ROESY and NOESY experiments. Phase-sensitive experiments are particularly difficult to process because some of the peaks are supposed to be inverted. Because I now have a sense of what the experiment is supposed to give, I have much more confidence in my ability to extract that information from my own experiments. The last section on advanced processing techniques, from weighting the FID to linear prediction, suggests that data sets I have thrown away because I felt that they were irrevocably flawed might have been salvageable. This is particularly crucial for my research, which involves NMR studies on unstable dications made from compounds that may have taken an undergraduate student weeks to make. If I can salvage some of the data, I have saved myself and my student a great deal of effort. I initially assumed that the appropriate audience for this book was the owner of a Bruker NMR spectrometer. However, as indicated in the beginning, the information appears transferable to a variety of spectrometers and I now feel that anyone who owns a high-field NMR spectrometer and who wants to exploit its full power should own this book. The learning curve for the processing of data from sophisticated NMR experiments is very steep. This book goes a long way toward giving the practical information needed to effectively use the data that the spectrometer can provide with such ease.

COSY spectroscopy of a set of four bicyclic[3.1.1]terpenes is described. The rigid carbon framework gives dihedral angles of nearly 90° for several adjacent protons which results in very small coupling constants for those protons. The rigid framework also results in W-coupling between the two non-adjacent bridgehead protons. The unexpected coupling which is shown clearly in the COSY spectrum can be illuminated by molecular modeling of the terpenes. Thus students learn both to use COSY spectroscopy to assign peaks in the 1H spectrum of a terpene but also to use the results from molecular mechanics calculations to understand the results from unusual geometries.

Metallation of 2-methyl-4-phenylbut-1-ene 1 resulted in the formation of a dianion linearly conjugated with the phenyl ring rather than the cross-conjugated dianion often seen in these systems. Examination of the reaction mixtures at varying periods of time revealed that the initial deprotonation was not at the benzylic carbon. Complexation of n-butyllithium–tetramethylethylenediamine (TMEDA) with the aromatic ring of 1(a complex-induced proximity effect, CIPE) enhances the rate of metallation at the carbon β to the aromatic ring and α to the double bond of 1, as demonstrated in competitive metallations of 1 with ethylbenzene and of 1 with hexa-1,5-diene. This is the first report of a complex induced proximity effect in the double metallation of a hydrocarbon in which complexation occurred with the neutral hydrocarbon.