A new class of spirocyclic imines (SCIs) has been theoretically investigated by applying a variety of quantum chemical methods and basis sets. The uniqueness of these compounds is depicted by various peculiarities, e.g., the incidence of planar six-membered rings each with two imine groups (two π bonds) and the incorporation of the isosteres carbon, silicon, or germanium spiro centers. Additional peculiarities of these novel SCIs are mirrored by their three-dimensionality, the simultaneous occurrence of nucleophilic and electrophilic centers, and the cross-hyperconjugative (spiro-conjugation) interactions, which provoke charge mobility along the spirocyclic scaffold. Substitution of SCIs with strong electron-withdrawing substituents, like the cyano group or fluorine, enhances their docking capability and impacts their reactivity and charge mobility. To gain thorough knowledge about the molecular properties of these SCIs, their structures have been optimized and various quantum chemical concepts and models were applied, e.g., full NBO analysis and the frontier molecular orbitals (FMOs) theory (HOMO-LUMO energy gap) and the chemical reactivity descriptors derived from them. For the assessment of the charge density distribution along the SCI framework, additional complementary quantum chemical methods were used, e.g., molecular electrostatic potential (MESP) and Bader’s QTAIM. Additionally, using the aromaticity index NICS (nuclear independent chemical shift) and other criteria, it could be shown that the investigated cross-hyperconjugated sila and germa SCIs are spiro-aromatics of the Heilbronner Craig-type Möbius aromaticity.

Dansylamide is perhaps the most ubiquitous fluorophore due to its donor-acceptor bifunctionality and its ability to form intra- and intermolecular hydrogen bonding. Among the diversity of its applications is the development of new generation of biosensors for the in vivo monitoring of traces of metals. The structure and conformational stability of dansylamide in the gas phase were investigated for the first time by a combined gas-phase electron diffraction-mass spectrometry (GED/MS), complemented by quantum chemical calculations. GED data indicate that different skewed conformers exist at T = 464 K, which are characterized by the deviation of two S–N bonds from the perpendicular orientation relative to the naphthalene plane. Maybe the most indicative structural parameters for electronic interactions between the donor-acceptor substituents and the aromatic naphthalene and the subsequent stabilization of the favorable skewed eclipsed-syn conformer are the dihedral angles C9–C1–S–N and C10–C5–N–C with the experimentally determined values of 66.8° (32) and 68.1° (72), respectively. The role of –SO2NH2 by forming intramolecular hydrogen bonds was scrutinized by employing the natural bond orbital approach (NBO), quantum theory atoms in molecules (QTAIM), and molecular electrostatic potential (MESP). The non-planarity of the naphthalene skeleton due to the electronic interactions with the substituents and its consequence for the fluorescence activity of dansylamide have been discussed.

The saturated vapors of 1- and 2-naphthalenesulfonamides (1-NaphSA and 2-NaphSA) were studied by the gas-phase electron diffraction/mass-spectrometric method at 413(9) and 431(9) K. According to quantum chemical calculations (DFT/B3LYP and MP2 with cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, and aug-cc-pVTZ basis set) 1-NaphSA possesses four conformers with different orientations of the SO2NH2 fragment relative to the naphthalene frame and eclipsed or staggered orientation of the N-H and S═O bonds, whereas 2-NaphSA possesses only two conformers with different orientations of the N-H and S═O bonds. It was experimentally established that vapors over 1-NaphSA and 2-NaphSA exist predominantly (up to 75 mol %) of low-energy conformers of C1 symmetry in which the C-S-N planes deviate from perpendicular orientation relative to the naphthalene skeleton with near eclipsed orientation of the N-H and S═O bonds of the SO2NH2 fragment. The following geometrical parameters (Å and degrees) of the dominant conformers were derived: r(h1)(C-H) = 1.089(4), r(h1)(C-C)av = 1.411(3), r(h1)(C-S) = 1.761(10), r(h1)(S-O)av = 1.425(3), r(h1)(S-N) = 1.666(10), ∠C-C1-C = 119.8(2), ∠C1-S-N = 104.5(22), C9-C1-S-N = 69.5(30) for 1-NaphSA; r(h1)(C-H) = 1.083(5), r(h1)(C-C)av = 1.411(3), r(h1)(C-S) = 1.780(7), r(h1)(S-O)av = 1.427(4), r(h1)(S-N) = 1.668(6), ∠C-C2-C = 123.0(3), ∠C2-S-N = 103.6(19), C1-C2-S-N = 110(10) for 2-NaphSA. The connection between nonequivalence of the C-C bonds in the naphthalene frame and spatial orientation of the substituents SO2NH2 is discussed. Transition states between conformers and enantiomers were determined.

α-Naphthalenesulfonyl chloride, α-NaphSC, was studied by gas-phase electron diffraction (GED) and quantum chemical calculations (HF/6-311 + G**, HF/aug-cc-pVDZ, B3LYP/cc-pVDZ, B3LYP/cc-pVTZ, B3LYP/aug-cc-pVDZ, B3LYP/aug-cc-pVTZ, MP2/cc-pVDZ, and MP2/cc-pVTZ). The calculations predict the existence of two conformers with C 1 (I) and C s (II) symmetries. The most stable conformer I has an enantiomer. The experimental data of α-NaphSC obtained at 370(5) K could be best fitted by a C 1 symmetry model indicating that only this form exists in the gas-phase. In this model the Cα–S–Cl plane deviates from the perpendicular orientation relative to the plane of the naphthalene skeleton. Under the applied experimental conditions, the mole fraction of a second less stable conformer II of α-NaphSC predicted by calculations is no more than 1 %. The following geometrical parameters of conformer I were obtained from the experiment (Å and °; uncertainties are in parentheses): r h1(C–H) = 1.082(6), r h1(C–C)cp = 1.407(3), r h1(C–S) = 1.764(5), r h1(S–O)av = 1.425(3), r h1(S–Cl) = 2.051(5), ∠C–Cα–C = 122.5(1), ∠Cα–S–Cl = 101.5(10); C9–C1–S–Cl = 71.4(21). The calculated barriers to internal rotation of the sulfonyl chloride group exceed considerably the thermal energy values corresponding to the temperatures of the GED experiments. Natural bond orbitals analysis of the electron density distribution was carried out to explain the peculiarities of the molecular structure of the studied compound and the deviation from the structures of β-NaphSHal molecules and their benzene analogs.

β-naphthalene sulfonyl fluoride, β-NaphSF, and β-naphthalene sulfonyl chloride, β-NaphSCl, were studied by gas-phase electron diffraction (GED) and quantum chemical calculations (B3LYP and MP2 in combination with cc-pVDZ, aug-cc-pVDZ and cc-pVTZ basis sets). For each compound the calculations predicted the existence of two conformers which are enantiomers. On the basis of the experimental data it was found that the gas phase over β-NaphSF and NaphSCl at 357(5)K and 395(5)K, respectively, consists of molecular species of C1 symmetry in which the CβSHal plane deviates from the perpendicular orientation relative to the naphthalene skeleton plane. The following geometrical parameters (Å and degrees) were obtained from the experiment (uncertainties are in parentheses): rh1(CH)aver.=1.097(7), rh1(CC)aver.=1.410(3), rh1(CS)=1.753(6), rh1(SO)aver.=1.414(4), rh1(SF)=1.559(5), ∠CCβC=122.8(3), ∠CβSF=103.3(30); Φ(CαCβSF)=104(6) for β-NaphSF, and rh1(CH)aver.=1.089(4), rh1(CC)aver.=1.411(3), rh1(CS)=1.757(5), rh1(SO)aver.=1.419(3), rh1(SСl)=2.053(4), ∠CCβC=122.8(1), ∠CβSCl=102.2(7), Φ(CαCβSCl)=108(3) for β-NaphSCl. The calculated barriers to internal rotation of the sulfonyl halide groups exceed considerably the thermal energy values corresponding to the temperatures of the GED experiments. Natural bond orbital (NBO) analyses of the electron density distribution were applied to explain the peculiarities of the molecular structure of the studied compounds and the deviation from the structures of their benzene analogs.

HF, B3LYP, and MP2 wave functions in combination with Pople 6-31, 6-311 augmented with polarization functions on all atoms and Dunning double- and triple-zeta basis sets have been employed to investigate the structures and torsional potential function of the nitro group in 2-nitropyridine- N -oxide (2-NPO) and a variety of its fluorinated derivatives. The augmentation of the basis sets with diffuse functions showed a marked effect on the profile and barriers of the NO 2 torsional potential. Depending on the applied model chemistry, the heterocyclic ring in some 2-NPOs has proved to be non-planar. The non-planarity of the ring was characterized by Cremer–Pople puckering amplitude Q . The disruption of the ring planarity in some NPOs was accounted for the distinctive reactivity and impact sensitivity of these heterocycles. Consistently, the NBO and the AIM analyses furnished clear evidence for the accentuated weakness of the C–NO 2 bond and provided evidence for the electronic interplay between the NO 2 group, the fluorine substituent and the heterocyclic ring. Deletion of all off-diagonal Fock-matrix elements (NOSTAR) to separate hyperconjugative stabilizing interactions from steric interactions was used. The effect of nitration and fluorination on the aromaticity of the studied 2-NPOs was investigated by using the NICS descriptors NICS(1) and NICS(1) zz . These NICS indices have shown that the fluorination in para position to the nitro group exhibits the highest degree of aromaticity within the fluorinated 2-NPOs.

The gas-phase electron diffraction experiment has shown that 1-monobromosilacyclobutane (MBSCB) exists in two conformational forms, the axial and equatorial with a significantly higher prevalence of the latter form (73(6)%). Various quantum mechanical procedures have been applied to investigate the thermodynamic equilibrium of the two conformers as well as the geometrical parameters of MBSCB. Among these methods was MP2/6-311++G(2df,2pd). This level of theory provided an axial to equatorial ratio of 29:71% and values for the geometrical parameters that are in good agreement with the experimental values except for the C–C bond length, which is by 0.02Å shorter than in the experiment. The main geometrical parameters obtained from the experiment are: (ra Å, ∠a°): Si–C 1.872(3); C–C 1.583(6); Si–Br 2.225(3); ∠CSiC 79.3; dihedral angle φ 28.8(52) (axial) and 39.9(2) (equatorial). Natural bond orbital (NBO) and atoms in molecule (AIM) analyses have been performed. Both, the donor–acceptor (Lewis and non-Lewis) orbital interactions and the topological properties of the charge density at the critical points ρ(r) have consistently confirmed the experimental results and facilitated their interpretation. For the purpose of comparison and systematic investigation, we optimized the geometries and analyzed the NBOs and the topological properties of silacyclobutane, 1-monofluorosilacyclobutane (MFSCB), and 1-monochlorosilacyclobutane (MCSCB). Simple relationship has been found between the puckering angle θ and the puckering amplitude q, which allows for the prediction of either θ or q for mono- and dihaloginated silacyclobutanes. Additionally, NBO deletion analysis comprising NOSTAR, NOVIC, and NOGEM deletion algorithms have been conducted. Interesting conclusions regarding structure and conformational stability of the studied monohalogenated silacyclobutanes could be drawn from this analysis.

Heinz Oberhammer was born on June 4th, 1939 in Innsbruck, Austria, where he attended school.After he graduated from high school in 1957 he studied physics and mathematics at the University of Innsbruck.He obtained his PhD in 1964 with a thesis on theoretical nuclear physics.Shortly after graduation he received an offer by a US government laboratory to continue working in the field of nuclear physics.The requirements for this job were an application for an immigration visa and commitment for at least 3 years.As everything was almost

The molecular structure and conformation of tris(cyclopropylsilyl)amine (TCPSA) has been studied by means of gas-phase electron diffraction at 338K and quantum-chemical calculations. A total of 12 relatively stable conformations of TCPSA molecule were considered. According to the experimental results and the DFT calculations the most stable conformer corresponds to a configuration (according to the Prelog–Klyne notation) of the type (−ac)(−ac)(+ac)-(−ac)(−ac)(+ac), where the first three parentheses describe the three different Si–N–Si–C torsional angles and the latter ones depict the rotation of the three cyclopropyl groups about the Cring–Si axes, respectively. The quantum-mechanical calculations were performed using various density functional (B3LYP, X3LYP and O3LYP) and perturbation MP2 methods in combination with double- and triple-ζ basis sets plus polarization and diffuse functions. The most important experimental geometrical parameters of TCPSA (ra Å, ∠h1 degrees) are: (Si–N)av=1.741(3), (Si–C)av=1.866(4), (C–C)av=1.510(3), (C–C(Si))av=1.535(3), (N–Si–C)av=115.1(18)°. For the purpose of comparison and searching for reasons leading to the planarity of the Si3N moiety in trisilylated amines we carried out NBO analysis and optimized the geometries of numerous silylamines. Among these compounds was tris(allylsilyl)amine (TASA), which is isovalent and isoelectronic to TCPSA. Utilizing the structural results we obtained we could show that Si+⋯Si+ electrostatic repulsive interaction is predominantly responsible for the planarity of the Si3N skeleton in TCPSA and in all other trisilylamines we considered. We also found that regardless the size and partial charges of the substituents the Si–N–Si bond angle in various disilylamines amounts to 130±2°.

Using Bader’s quantum-topological theory of atoms in molecules (AIM) and Weinhold’s Natural Bond Orbital (NBO) analysis we could rationalize the impact of the geminal substitution by C≡N and Cl on the geometry and electronic structure of the cyclopentane ring in 1,1-dicyanocyclopentane (DCCP) and 1,1-dichlorocyclopentane (DClCP). Among the crucial results we obtained are: 1. The topological quantities, particularly the bond ellipticity of 0.035 for the C–CN bond, indicate that this bond possesses higher bond order than a single bond. This conclusion is clearly supported by the NBO results. 2. The AIM theory as well as the NBO analysis confirm uniformly the non-linearity of the C–C≡N moiety. 3. Regardless the quantum mechanical method that has been employed for a variety of nitriles the sign of the Laplacian of the charge density, ∇2ρ(r), of the C≡N group changes its sign from negative to positive upon moving from the triple zeta to the double zeta basis set. A possible explanation for this striking behavior has been provided. By invoking the AIM and NBO approaches the different endocyclic C–C bond lengths in DCCP and DClCP as a consequence of the geminal substitution could be explained. Also the variations of these bond lengths upon moving from the more stable C s to the energetically less favorable C 2 conformer of both compounds could be rationalized. For the purpose of comparison and verification of some findings of this work, we also carried out AIM and NBO calculations on various related cyclic and non-cyclic compounds.

Encouraged by the results we recently obtained from the exploration of the dependency of the structural parameters of 1,1-dichlorocyclopentane (J. Chem. Phys. A 2004, 108, 4658) on the pseudorotational parameter phi, we decided to reinvestigate the structure and the potential function governing the conformational equilibrium of 1,1-dicyanocyclopentane (DCCP) in the light of these novel results. The improved potential function we developed describes more adequately the dependency of the geometrical parameters on the pseudorotational phase angle phi. In the present work, we also incorporated additional terms into the equations we developed earlier (J. Chem. Phys. A 2004, 108, 4658; J. Mol. Struct. 2002, 612, 181) for describing the dependency of the distribution of the delocalized net charges throughout the ring on phi to account for the observed systematic deviations between the computed atomic distances and those provided by these equations. Although the overall fit of the electron diffraction was not significantly different from that which we presented previously, however, applying these extended equations has led to a better fit by refining a smaller number of parameters.

The molecular structure of fluoromalononitrile was studied by means of gas-phase electron diffraction and quantum mechanical methods using HF/6-31G(d), MP2/6-311++G(2df,2pd) and DFT/B3LYP/6-31G(d), B3PW91/6-31G(d), B3LYP/6-311++G(2df,2pd) and B3PW91/6-311++G(2df,2pd). The rg and ∠α structural parameters we obtained from the present analysis are: CC = 1.487(5) Å, CN = 1.157(3) Å, CF = 1.386(5) Å, CH = 1.096 Å (ass.), ∠CCC = 106.7(1.0)°, ∠CCF = 108.0(0.7)°, ∠CCN = 177.6(2.0)°. Uncertainties in parenthesis are 3σ.

The molecular structure of 1,1-dichlorocyclopentane (DClCP) has been investigated by means of gas-phase electron diffraction and ab initio calculations. Although the electron diffraction data could be fairly well reproduced by a C, (envelope) model we found it more pertinent to apply a pseudorotation model to account for the dynamic and large amplitude motion in DClCP. On the basis of this model we analyzed the dependency of the ring geometric parameters and vibrational mean amplitudes on the phase angle phi. For a better elucidation of this distinct dependency we developed particular equations which describe the dependency of the distribution of the delocalized net charges throughout the ring on the phase angle phi. The joint electron diffraction and ab initio study has led to the following r(a) structural parameters of DClCP (C-s conformer): r(C-Cl)(ax) = 1.787(3)Angstrom, r(C-Cl)(eq) = 1.769(3) Angstrom, average r(C-C)(ring) = 1.535(1) Angstrom, r(C-H)(av) = 1.114(5) Angstrom, angle(C5 - C1 - C2) 103.0(9)degrees, angle(Cl - C - Cl) = 108.6(3)degrees, and angle(H-C-H) = 104.6(26)degrees. The puckering amplitude for the five-membered ring was determined to be q = 0.400(11) Angstrom. The quantum mechanical calculations were carried out by utilizing the Hartree-Fock, density functional B3PW91, and perturbation MP2 methods and applying the following basis sets: cc-pVDZ, 6-31G(d,p), 6-311G(df,pd), 6-311+G(d,p), 6-311++G(d,p), 6-311+ G(2df,2pd), and 6-311++G(2df,2pd). For the purpose of comparison and systematic study, we optimized the geometries and calculated the charge distributions using the natural population analysis (NPA) and Mulliken population analysis (MPA) of 1,1-difluorocyclopentane, 1,1-dibromocyclopentane, and their corresponding monohalogenated derivatives.

The molecular structure of magnesium bis-acetylacetonate has been determined by synchronous gas-phase electron diffraction (GED) and mass spectrometric experiment and quantum mechanical calculations (HF/6-311+G(d,p), B3LYP/6-311+G(d,p), and MP2/6-311+G(d,p)). Both experimental and theoretical approaches yielded the structure of D2d symmetry with chelate rings located in perpendicular planes. Major structural parameters determined by GED experiment are the following: bond distances rα(Mg–O)=1.966(4), rα(O–C)=1.279(2), rα(C–C′)=1.408(3), rα(C–Cm)=1.534(4)Å; valence angles ∠α(O–Mg–O)=93.3(2), ∠α(O–C′–C)=125.3(2), ∠α(O–C–Cm)=116.2(4)°.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTSuperoxygenated Water as an Experimental Sample for NMR RelaxometryMarwan Dakkouri , Hubert Rauscher , and Nikolaus Nestle View Author Information Department of Electrochemistry, University of Ulm, D-89069 Ulm, Germany Department of Surface Chemistry and Catalysts, University of Ulm, D-89069 Ulm, Germany Institute of Hydrochemistry, Technische Universität München, D-81377 München, GermanyCite this: J. Chem. Educ. 2004, 81, 7, 1040Publication Date (Web):July 1, 2004Publication History Received3 August 2009Published online1 July 2004Published inissue 1 July 2004https://pubs.acs.org/doi/10.1021/ed081p1040https://doi.org/10.1021/ed081p1040research-articleACS PublicationsRequest reuse permissionsArticle Views213Altmetric-Citations3LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose SUBJECTS:Magnetic resonance imaging,Molecules,Nuclear magnetic resonance spectroscopy,Oxygen,Students Get e-Alerts

The molecular structure of 1,1-dicyanocyclopentane (DCCP) has been investigated by means of gas-phase electron diffraction and ab initio calculation. Although the electron diffraction data could be fairly good reproduced by a Cs (envelope) model we found it more pertinent to apply a pseudorotation model to account for the dynamic and large amplitude motion in DCCP. Based on this model we analyzed the dependency of the ring geometric parameters and vibrational mean amplitudes on the phase angle φ. For a better elucidation of this distinct dependency we developed particular equations which describe the dependency of the distribution of the delocalized net charges throughout the ring on the phase angle φ.For the purpose of a more systematical study of the substituent effect exerted by the cyano group we also investigated the structure and conformational stability of monocyanocyclopentane (MCCP) by means of various quantum mechanical methods. The MP2 method in combination with the basis sets 6-311+G(2df,2pd), 6-311++G(d,p), and 6-31G(d,p) favors the axial conformer which is in contradiction to earlier results which were obtained from electron diffraction data [J. Mol. Struct., 116 (1984) 29]. Using the same basis sets but the DFT/B3PW91 method, however, leads to a more stable equatorial conformer. This striking behavior is discussed in this paper.The joint electron diffraction and ab initio study has led to the following rα structural parameters of DCCP: r(CN)=1.152(2)Å, r(C–C)=1.472(5)Å, average r(C–C)ring=1.549(3)Å, ∠(C5–C1–C2)=103.6(26)°, ∠(NC–C–CN)=109.0(35)°, ∠(C–CN)=175.2(33)°, and ∠(H–C–H)=112.7(23)°. The puckering amplitude for the five-membered ring was determined to be q=0.434(45)Å.The quantum mechanical calculations were carried out by utilizing the Hartree–Fock, density functional B3PW91, and perturbation MP2 methods and applying the basis sets: 6-31G(d,p), 6-311G(df,pd), 6-311+G(d,p) and 6-311++(2df,2pd).In contrast to the Mulliken poulation analysis the natural population analysis provided clear evidence for the electronic interaction and bond conjugation within the geminally substituted cyano groups.

As a follow-up on our previous study of a series of purines (purine, 6-chloropurine, purine-6-thiol, hypoxanthine, theobromine, theophylline, caffeine, and uric acid), we have investigated six additional biologically important purines (adenine, guanine, isoguanine, thioguanine, xanthine, and kinetin). Their ground-state dipole moments were measured in dioxane at 293 K. The first excited singlet-state dipole moments were obtained using the solvatochromic shift equations (McRae, Suppan, Bakhshiev, and Kawski-Chamma-Viallet). The theoretical dipole moments were calculated as a combination of the π-moment (PPP method) and the σ-moment (vector sum of the σ-bond and σ-group moments). The same approach was used to obtain their first excited singlet-state dipole moments (excited state π-moment; σ-moment assumed to be the same as in the ground state).Ab initioHF 6-31G**calculations were also used to obtain ground-state dipole moments for all the fourteen purines under study. In addition, a DFT/B3PW91/6311++(2df,2p) calculation has been carried out for purine for comparison. The different sets of theoretical dipole moments were compared with the respective experimental values. There is an approximately equally good agreement among the experimental dipole moments and the PPP + σ dipole moments (±6.9%) and theab initiodipole moments (±7.4%). The effect of structure on the dipole moments is discussed.