Sp Conformation Research Articles

The barrier to internal rotation in silabutenes: A density functional theory study

The potential energy surfaces (PESs) for internal were systematically investigated by using DFT with the B3LYP function. QTAIM, NBO, NEDA and BLW-ED were employed to study the origin of rotational barriers. The silabutenes with M1 = C–M2–M3 skeleton preferred the ac conformer and the M1 = Si–M2–M3 skeletons have the sp conformer. Silabutenes including SiSi double bond have three stable conformers, two different ac conformer and one gauche conformer. Internal rotation barriers (0.38–2.47 kcal/mol) vary with bond lengths (SiC, SiSi, CC), decomposed into steric, electrostatic, and hyperconjugation terms using NSA, NEDA, and NBO. A linear correlation (R2 = 0.78) links barrier heights to energetic differences between stable and metastable conformers. Understanding these factors aids in predicting and rationalizing rotational barriers in silabutenes, crucial for their chemical behavior and applications.

"First Come, First Served" and Threshold Effects in a Central-to-Planar-to-Helical Hierarchical Chiral Induction.

Perylene diimide tethered pillar[5]arene derivatives form aggregates in non-polar organic solvents, and the complexation of cationic amino acid ethyl ester (cAA-OEt) with the aggregates induce a central-to-planar-to-helical chirality transfer, leading to intensive circular dichroism (CD) signals having dissymmetric factors (gabs ) of up to 3.67×10-2 . The hierarchical chiral induction exhibited an intriguing threshold dose effect, namely, the chiral induction does not occur in the low concentration range of cAA-OEt but is triggered when cAA-OEt exceeds a threshold concentration. The inhibited interconversion between the Rp and Sp conformers of pillar[5]arene, which is further restricted in the aggregation, plays a crucial role in the threshold effect. When adding enantiopure cAA-OEt first to the threshold concentration and then adding an equal amount of the antipodal cAA-OEt to give cAA-OEt in racemic form, CD spectra having the same sign as the CD induced by first adding pure cAA-OEt were induced, thus showing an unprecedented "first come, first served" effect.

“First Come, First Served” and Threshold Effects in a Central‐to‐Planar‐to‐Helical Hierarchical Chiral Induction

AbstractPerylene diimide tethered pillar[5]arene derivatives form aggregates in non‐polar organic solvents, and the complexation of cationic amino acid ethyl ester (cAA‐OEt) with the aggregates induce a central‐to‐planar‐to‐helical chirality transfer, leading to intensive circular dichroism (CD) signals having dissymmetric factors (gabs) of up to 3.67×10−2. The hierarchical chiral induction exhibited an intriguing threshold dose effect, namely, the chiral induction does not occur in the low concentration range of cAA‐OEt but is triggered when cAA‐OEt exceeds a threshold concentration. The inhibited interconversion between the Rp and Sp conformers of pillar[5]arene, which is further restricted in the aggregation, plays a crucial role in the threshold effect. When adding enantiopure cAA‐OEt first to the threshold concentration and then adding an equal amount of the antipodal cAA‐OEt to give cAA‐OEt in racemic form, CD spectra having the same sign as the CD induced by first adding pure cAA‐OEt were induced, thus showing an unprecedented “first come, first served” effect.

Chiroptical regulation of macrocyclic arenes with flipping-induced inversion of planar chirality.

Studies on various macrocyclic arenes have received increasing attention due to their straightforward syntheses, convenient derivatization, and unique complexation properties. Represented by pillar[n]arenes, several distinctive macrocyclic arenes have recently emerged with the following characteristics: they possess a pair of enantiomeric planar chiral conformations, and interconversion between these enantiomeric conformations can be achieved through the flipping of ring units. Complexation of a chiral guest with these macrocyclic arenes will lead to a shift of the equilibrium between the Rp and Sp conformers, leading to intriguing possibilities for chiral induction and sensing. By the introduction of bulky substituents on the rims, employing rotaxanation or pseudocatenation, planar chirality could be locked, enabling the enantiomeric separation of the chiral structures. The induced or separated chiral conformers/compounds exhibit significant chiroptical properties. These macrocyclic arenes, with flipping-induced inversion of planar chirality, demonstrated intriguing chiral induction dynamics and kinetics. In this featured review, we systematically summarize the progress in chiroptical induction/regulation of these macrocyclic arenes, particularly in the fields of chiral sensing, molecular machines, molecular recognition, and assembly.

The More the Slower: Self-Inhibition in Supramolecular Chirality Induction, Memory, Erasure, and Reversion

Self-inhibition has been observed widely in hierarchical biochemical processes but has yet to be demonstrated in pure molecular physical rather than chemical or biological processes. Herein, we report an unprecedented example of self-inhibition during the supramolecular chirality induction, memory, erasure, and inversion processes of pillar[5]arene (P[5]) derivatives. The addition of chiral alanine ethyl ester to bulky substituent-modified P[5]s led to time-dependent chirality induction due to the shift in the equilibrium of the SP and RP conformers P[5]. Intriguingly, more chiral inducers led to more intensive final chiroptical properties but lower chiral induction rates. Thus, the chiral inducer plays the role of both activator and inhibitor. Such self-inhibition essentially arises from kinetics manipulation of three tandem equilibria. Moreover, the chiroptical properties could be memorized by replacing the chiral inducer with an achiral competitive binder, and the chiroptical signal could be erased and reversed by an antipodal chiral inducer, which also showed the self-inhibition property.

The two-top molecule 3-penten-2-one: Acetyl methyl torsion in α,β-unsaturated ketones

Using a molecular jet Fourier transform spectrometer, the microwave spectrum of E-3-penten-2-one, also called methyl propenyl ketone, was recorded in a range from 2.0 to 26.5 GHz. Two conformers, antiperiplanar (ap) and synperiplanar (sp), could be identified. Complicated splitting patterns arising due to the internal rotation of the acetyl methyl and propenyl methyl groups could be resolved, and all measured rotational transitions were fitted to measurement accuracy. The barrier heights of the acetyl methyl group are 434.149(37) cm−1 and 358.076(26) cm−1 for the ap and sp conformers, respectively. For the propenyl methyl group, the barrier is 581.903(45) cm−1 for the ap conformer and 595.271(71) cm−1 for the sp conformer. By comparing these results to those of other ketones categorized in a class system connecting the internal rotation of the acetyl methyl group to the structures of the molecules, we extended the categorization with a new class for α,β-unsaturated ketones.

Spectroscopic and theoretical studies of some 2-(2′-haloacetyl)-5-substituted: 1-Methylpyrrole, furan and thiophene

The conformational study of some 2-(2′-haloacetyl)-5-substituted five-membered heteroaromatic compounds Z-CH2C(O)(C4H2X)Y bearing a halogen (Z = Cl or Br) at the 2′-position and (Y = NO2, H, Me or Cl) substituents at the 5-position of the heteroaromatic ring for X = NMe, O and S was performed by IR carbonyl stretching band (νCO) analysis supported by the harmonic frequency, NBO and PCM calculations on the minima and saddle-point structures obtained at the M05-2X/aug-cc-pVTZ level. The computational results predicted the presence of four stable conformers, classified by the orientation of the Xring–CCO (syn/anti) and the OCC–Z (anticlinal/ synperiplanar) moieties as ac(syn), sp(syn), ac(anti) and sp(anti). The anti/syn equilibrium controls the relative stabilities of the conformers to a large extent while the anticlinal/synperiplanar accounts for the observed trend of the IR carbonyl band components. The computed νCO frequencies and the PCM trend of the relative abundance in solvents of increasing permittivity allowed the pairs of ac(syn)/ac(anti) and sp(syn)/sp(anti) conformers to be assigned to the lower and higher νCO frequency IR carbonyl doublet components, respectively. The PCM data failed to reproduce the experimental carbonyl triplet found in chloroform. However, the additional component could be justified by assuming the formation of complexes constituted by one chloroform molecule hydrogen bonded to the conformers in a PCM chloroform continuum.The NBO and short contact analysis suggested that the syn/anti conformational equilibrium is governed mainly by electrostatic interactions, while the anticlinal/synperiplanar depends to different extents on both orbital and electrostatic interactions.

Controlled modulation of the helical sense and the elongation of poly(phenylacetylene)s by polar and donor effects

The elasticity (stretched/compressed) and helical sense (clockwise/anticlockwise) of a dynamic helical poly(phenylacetylene) (PPA), bearing (R)-α-methoxytrifluorophenyl acetic acid (MTPA) pendants [poly-(R)-1], can be selectively controlled by the donor and the polar character of the solvent. The basis for this effect lies on the presence in the pendants of two independently tuneable bonds: (O)C–C(–O) and (H–)N–C(O). An increase in polarity shifts the conformational equilibrium of the (O)C–C(–O) bond in favour of the most polar synperiplanar (sp) conformer, that is more sterically demanding, thus forcing the polymer to invert its helix. A donor solvent associates with the (H–)N–C(O) bond favouring the shift to the cis form of the amide and inducing both the elongation of the polymer chain and the inversion of the helical sense (visualized as UV-Vis bathochromic shift and CD inversion respectively). This ability of poly-(R)-1 to respond to donor/polar properties occurs in solution as well as in film state. Poly-(R)-1 presents in CHCl3 identical helical sense for both the internal (polyene backbone) and for the external (described by the pendants) helices (3/1 clockwise), while in THF their senses are opposite (internal anticlockwise; external 2/1 clockwise), as proven by DSC.

Synthesis, calcium-channel blocking activity, and conformational analysis of some novel 1,4-dihydropyridines: application of PM3 and DFT computational methods

Novel 1,4-dihydropyridines were synthesized and subjected to calcium-channel blocking evaluation and conformational analysis using semi-empirical (PM3) and density functional theory (DFT) as computational methods. All molecules had a boat-like 1,4-dihydropyridine ring in both the methods. In PM3 method almost 54% of the molecules were deviated from planarity, but in DFT method all the molecules had perfect flattened-boat conformation. Using both the methods, the C-4 substituent was pseudoaxial with its phenylamino substitution in sp orientation in 82.14% of the molecules. Trans–trans and cis–cis conformation had the greatest and lowest proportion in the molecules, respectively. Trans–trans conformers which possessed sp conformation for the substituted group on the imidazole ring were active calcium-channel blocking agents.

High Resolution Millimeter-Wave Spectroscopy of Vinyltellurol

The millimeter-wave rotational spectrum of vinyltellurol has been recorded and assigned for the first time. To support the spectrum assignment, high level ab initio calculations have been carried out. Geometries, total electronic energies, and harmonic vibrational frequencies have been determined at the MP2 level. A small-core relativistic pseudopotential basis set (cc-pVTZ-PP) was employed to describe the tellurium atom. Two stable conformers, synperiplanar (sp) and anticlinal (ac), have been identified. The sp conformer is planar with a small negative inertia defect of -0.025 u Å(2). The ac conformer was found to be nonplanar with a C-C-Te-H dihedral angle of about 140° from sp. This conformer exhibits a large amplitude motion associated with the torsion about the C-Te bond. The barrier to internal rotation is about 1 kJ/mol, according to the theoretical calculations. For the ac conformation, a torsional potential function consisting of quartic and quadratic terms of the torsional angle has been partially determined from the observed rotational constants.

4-Nitro-2-phenoxyaniline

In the title compound, C12H10N2O3, the oxygen atom bridging the two aromatic rings is in a synperiplanar (+sp) conformation. The dihedral angle between the aromatic rings is 71.40 (12)°. In the crystal, mol­ecules are linked by inter­molecular N—H⋯O hydrogen bonds.

Structure, Conformation, Stereodynamics, Dimer Formation, and Absolute Configuration of Axially Chiral Atropisomers of Hindered Biphenyl Carbinols

NMR spectra of biphenyl derivatives bearing a single CR2OH substituent in the ortho position indicate that they exist as sp (more stable) and ap (less stable) conformers, due to the restricted rotation about the Ar-CR2OH bond. When R = Et (compound 2) the corresponding rotation barrier was determined (7.5 kcal mol-1) by line shape simulation of the low-temperature NMR spectra. Introduction of the prochiral i-Pr group in the position 3' of a biphenyl with the CMe2OH substituent in the position 2 (4) allowed the determination of the enantiomerization barrier (due to the Ar-Ar bond rotation) for the stereolabile axially chiral atropisomers (13.95 kcal mol(-1)). DFT computations of these barriers were all in agreement with the experiments. Biphenyls bearing two CR2OH groups in the 2,2' positions were found to exist as configurationally stable atropisomers: when R = Me (7) they were separated by enantioselective HPLC and the absolute configuration assigned on the basis of the corresponding CD spectra. In solution, compounds 6 (R = H) and 7 (R = Me) were found to originate a dimer, due to H-bond interactions between two enantiomers. In the case of 7, the free energy of activation (9.5 kcal mol-1) for the exchange of the monomer with the dimer could be measured, for the first time, by dynamic NMR. The conformational preferences, predicted by computations for the biphenyls with two CR2OH substituents in the 2,2' positions, were confirmed by X-ray diffraction in the case of R = H (6), R = Me (7), and R = i-Pr (9).

Experimental and theoretical study of two novel thioperoxides: Methoxycarbonylsulfenyl acetate, CH 3OC(O)SOC(O)CH 3, and methoxycarbonylsulfenyl trifluoroacetate, CH 3OC(O)SOC(O)CF 3

Two novel thioperoxides, methoxycarbonylsulfenyl acetate, CH 3OC(O)SOC(O)CH 3, and methoxycarbonylsulfenyl trifluoroacetate, CH 3OC(O)SOC(O)CF 3 were generated through heterogeneous reactions and studied by photoelectron spectroscopy (PES) and quantum chemical calculations. The prevailing components in the gas phase may be the gauche (sp–sp–sp) conformers, which exhibit gauche conformation around the S O bond and the C O bonds syn to the S O bond. The torsional angle δ CSOC of CH 3OC(O)SOC(O)CH 3 and CH 3OC(O)SOC(O)CF 3 are theoretically predicted to be 73.0° and 71.9° (MP2/6-311++G(d,p)), respectively. The first vertical ionization energies of CH 3OC(O)SOC(O)CH 3 and CH 3OC(O)SOC(O)CF 3 are determined to be 9.49 and 10.17 eV, respectively. According to the experimental results and theoretical analysis, the HOMOs of these two molecules both correspond to the electrons mainly localized on the sulfur 3p lone pair MOs.

Molecular structure of cotinine studied by gas electron diffraction combined with theoretical calculations

The molecular structure of cotinine (( S)-1-methyl-5-(3-pyridinyl)-2-pyrrolidinone), the major metabolite of nicotine, has been determined at about 182 °C by gas electron diffraction combined with MP2 and DFT calculations. The diffraction data are consistent with the existence of the (ax, sc), (ax, ap), (eq, sp) and (eq, ap) conformers, where ax and eq indicate the configuration of the pyrrolidinone ring by means of the position (axial and equatorial) of the pyridine ring, and sc, sp and ap distinguish the isomers arising from the internal rotation around the bond connecting the two rings. The (CH 3)NCCC(N) dihedral angles, ϕ, of the (ax, sc) and (eq, sp) conformers were determined independently to be 158(12)° and 129(13)°, respectively, where the numbers in parentheses are three times the standard errors, 3 σ. According to the MP2 calculations, the corresponding dihedral angles for the (ax, ap) and (eq, ap) conformers were assumed to differ by 180° from their syn counterparts. The ratios x(ax, sc)/ x(ax, ap) and x(eq, sp)/ x(eq, ap) were taken from the theoretically estimated free energy differences, Δ G, where x is the abundance of the conformer. The resultant abundances of (ax, sc), (ax, ap), (eq, sp) and (eq, ap) conformers are 34(6)%, 21% (d.p.), 28% (d.p.), and 17% (d.p.), respectively, where d.p. represents dependent parameters. The determined structural parameters ( r g (Å) and ∠ α (°)) of the most abundant conformer, (ax, sc), are as follows: r(N C) pyrrol = 1.463(5); r(N C methyl) = 1.457(←); r(N C( O)) = 1.384(12); r(C O) = 1.219(5); 〈 r(C C) pyrrol〉 = 1.541(3); r(C pyrrol C pyrid) = 1.521(←); 〈 r(C C) pyrid〉 = 1.396(2); 〈 r(C N) pyrid〉 = 1.343(←); ∠(CNC) pyrrol = 113.9(11); ∠CCC pyrrol(−C pyrid) = 103.6(←); ∠NCO = 124.1(13); ∠NC pyrrolC pyrid = 113.1(12); ∠C pyrrolC pyrrolC pyrid = 113.3(←); ∠(CNC) pyrid = 117.1(2); 〈∠(NCC) pyrid〉 = 124.4(←); ∠C methylNC( O) = ∠C methylNC(−C pyrid) = 122.8(d.p.); ∠NC( O)C = 107.1(d.p.); ∠NC pyrrol(−C pyrid)C pyrrol = 103.0(d.p.) and ∠CCC( O) = 105.2(d.p.), where ← in the parentheses means that the parameter is bound to the preceding one and 〈 〉 denote average values. The puckering angle, α, of the pyrrolidinone ring is 26(3)°. The N⋯N distances of the (ax, sc) and (eq, sp) conformers, which are 4.844(5) and 4.740(5) Å, respectively, are close to that of the most stable conformer of nicotine, 4.885(6) Å and the corresponding one of arecoline, 4.832(13) Å. It is concluded that the weak nicotinic activity of cotinine cannot be ascribed to such a small difference in the N⋯N distances.

The molecular structure of N-fluorobis(trifluoromethanesulfonyl)imide, NF(SO2CF3)2, as studied in the gas phase by electron diffraction restrained by ab initio calculations

The structure of N-fluorobis(trifluoromethylsulfonyl)imide, prepared by a relatively safe and easy method, has been determined by gas-phase electron diffraction (GED), employing the SARACEN method, with flexible restraints based on the MP2/6-311G* structure, and by X-ray crystallography at 150 K. The strongly electron-withdrawing CF(3) and SO(2)CF(3) groups make the C-S and N-S distances long, averaging 187.7(3) and 171.7(3) pm, respectively, in the gas phase. The gas consists of two conformers, one (75%) with a CF(3) group on each side of the SNS plane, one anti-periplanar and one syn-periplanar to the further N-S bond (ap, sp), and the other with both CF(3) groups on the same side, i.e. denoted ap, ap. These conformers have very different SNS angles, 126.9(9) degrees and 117.1(17) degrees respectively. In the crystal all molecules have the ap, sp conformation, with parameters similar to those found for this conformer in the gas phase.

Theoretical study of the structure of (O→Si)-(acetoxymethyl)trifluorosilane dimers

According to the data of quantum chemical calculations (HF, DFT), the isolated sp,sp conformer of (acetoxymethyl)trifluorosilane — pentacoordinated silicon compound — forms dimers with two intermolecular bonds Si-F→Si. The =O→Si intramolecular coordination bonds become stronger, which leads to the general stabilizing effect. In polar media, dimer formation is energetically unfavorable relative to monomer formation.

Synthesis, Structure and Conformational Properties of Fluoroformylchlorodifluoroacetyl Disulfide, FC(O)SSC(O)CF2Cl: Conformational Transferability in –C(O)SSC(O)– Compounds

AbstractPure fluoroformylchlorodifluoroacetyl disulfide, FC(O)SSC(O)CF2Cl, has been prepared by the reaction of FC(O)SCl and CF2ClC(O)SH in quantitative yield. The conformational properties of the novel molecule have been studied by vibrational spectroscopy (IR – gas phase, Raman – liquid phase) and quantum chemical calculations (B3LYP and MP2 methods). The gaseous compound exhibits a conformational equilibrium at room temperature where the most stable form adopts a C1 symmetry and a syn‐periplanar (sp) orientation of both carbonyl groups with respect to the disulfide bond. A second form, observed in the IR spectrum of the vapor, corresponds to a conformer in which the carbonyl bond of the FC(O) moiety adopts an anti‐periplanar (ap) position with respect to the S–S single bond, and gauche with respect to the ClC–C=O moiety in the chlorodifluoroacetyl group. The experimental free energy difference value ΔG0 = G0(ap–sp) – G0(sp–sp) = 1.4(3) kcal/mol (IR) is reproduced well by the B3YLP/6‐311+G(3df) (1.1 kcal/mol) and the MP2/6‐31G* (1.8 kcal/mol) methods. In addition, the structure of a single‐crystal, grown in situ, was determined by X‐ray diffraction analysis at low temperature. The crystalline solid [monoclinic, P21/n, a = 5.579(3) Å, b = 16.615(7), c = 8.455(4) Å, β = 106.876(8)°] consists exclusively of molecules with the (sp–sp) conformation and the usual gauche orientation around the disulfide bond [φ(CS–SC) = 84.2°]. Conformational transferability is thus demonstrated once again for species that contain the –C(O)SSC(O)– group as part of a systematic program designed to analyze the behavior of this class of molecules. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Unveiling the “Booster Effect” of Fluorinated Alcohol Solvents: Aggregation-Induced Conformational Changes and Cooperatively Enhanced H-Bonding

The influence of conformation and aggregation on the hydrogen bond donor ability of fluorinated alcohol solvents [1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and 1-phenyl-2,2,2-trifluoroethanol (PhTFE)] was explored theoretically (DFT) and experimentally (NMR, kinetics, crystal structure analyses). The detailed DFT analysis revealed a pronounced dependence of the H-bond donor ability on the conformation along the CO-bond of the monomeric alcohols. The donor orbital energy (sigma*(OH)) decreases and the molecular dipole moment (mu) increases drastically from the antiperiplanar (ap) to the synperiplanar (sp) H(C)COH conformation. The kinetics of olefin epoxidation with H(2)O(2) in HFIP indicate higher order solvent aggregates (2-3 monomers) to be responsible for the activation of the oxidant. Single-crystal X-ray analyses of HFIP and PhTFE confirmed the existence of H-bonded aggregates (infinite helices, ribbons, and cyclic oligomers) and the predominance of sc to sp conformations of the fluoroalcohol monomers. These aggregate structures served as the basis for a DFT analysis of the H-bond donor ability at the terminal hydroxyl group of HFIP mono- to pentamers. Both the LUMO energy and the natural charge of the terminal hydroxyl proton indicated a substantial cooperative influence of dimerization and trimerization on the H-bond donor ability. We therefore conclude that dimers and trimers, with the individual monomers in their sc to sp conformation, play a crucial role for the solvolytic and catalytic effects exerted by HFIP, rather than monomers.

2-Acetylpyridine 1,1-(hexane-1,6-diyl)thiosemicarbazone-κ3N,N′,S]chloroplatinum(II)

In the title compound, [Pt(C14H19N4S)Cl], the azepane ring is partially disordered over two sites. The Pt atom coordination adopts a slightly distorted square-planar geometry. The 2-acetyl­pyridine thio­semicarbazone moiety is very close to planar. The azepane ring has a (−sc, +sc)3, +sp conformation (sc is synclinal and sp is synperiplanar) and, for the minor disorder component, a (−sc, +sc)3, −sp conformation. The computed bond valences for the Pt centre are νPt—N = 0.467 and 0.587, νPt—S = 0.60, and νPt—Cl = 0.48 v.u. (valence units), and thus the computed valence of the Pt atom is 2.13 v.u. π–π stacking inter­actions give rise to π-bonded dimers.

Quantitative evaluation of resonance interaction: monosubstituted 1,3-butadienes

Relative resonance energy of various groups X was expressed as the reaction energy of the isodesmic homodesmotic reaction in which this group is transferred from an aliphatic compound, 1-substituted butane or pentane, to 1-substituted butadiene or benzene. Energies of 18 ( E)-1-substituted 1,3-butadienes with ap or sp conformation on the central bond, and of the same number of substituted benzenes were calculated within the framework of the density functional theory at the level B3LYP/6-311+G(d,p). The reaction energies are very similar for all reaction series; in the case of ap-butadienes the reaction energy can be suggested as the most trustworthy standard scale of the substituent conjugative power (resonance). This scale differ significantly from the traditional scales of resonance effects, σ R , σ R + or σ R − , derived essentially from acid–base equilibria; the reason is that the resonance scales are influenced by the presence of charge. Relation of our model to the classical term of resonance was confirmed both from the C2–C3 bond lengths and from the Mulliken charges on the CH 2 group. The C2–C3 bond is shortened by all substituents, the charges are changed in the opposite direction by acceptors and donors. However, there is no direct proportionality between the substituents effects on the energy, on geometry and the charges.