Achieving Accuracy and Economy for Calculating Vertical Detachment Energies of Molecular Anions: A Model Chemistry Composite Methods.

The vertical detachment energy (VDE) is a vital factor for predicting the stability of anions that have important applications in the atom, molecule and cluster science. Due to the synthetic or characterization difficulty of anions, accurate and efficient predictions of VDE independent of laboratory data have always been an appealing task to remedy the experimental deficiencies. Unfortunately, the generally adopted CCSD(T) and electron propagator theory (EPT) methods have respectively been proven to be reliable but very cost-expensive, and cost-effective but sometimes problematic when Koopman's theorem is invalid. Here, we for the first time introduced and benchmarked a series of model chemistry composite methods (e. g., CBS-QB3, G4 and W1BD) on calculating VDE for 57 molecular anions. Notably, CBS-QB3 exceeds the accuracy of CCSD(T) while approaching the economy of EPT. Therefore, we highly recommend the composite method CBS-QB3 to compute VDEs for molecular anions in the attractive "killing two birds with one stone" manner.

Building a New Platform for Significantly Improving Performance of Hartree-Fock and CCSD(T) Correlation Energy Based on Two-Point Complete Basis Set Extrapolation Schemes.

The leading cause of high expense in gold standard coupled cluster theory is that calculations of electronic energies converge exceedingly slowly with an increased basis set size. Extrapolation principally allows for achieving higher-quality outcomes at reduced costs. Numerous extrapolation formulas have been developed, with attempts to predict energies up to the complete basis set limit. Unfortunately, since the intricate shape of the function hinges on the molecular properties with the highest angular momentum of the basis set, the accuracy of the extrapolated energies highly depends on the fitted empirical parameters, which rely on the quality of the data sets for fitting. In this work, to overcome the extrapolation deficiency caused by the very limited data sets and smaller basis sets in the early stages, we constructed a new benchmark platform that includes a broader data set of 183 species (containing open-shell, closed-shell, ionic, and neutral species) and a larger basis set up to aug-cc-pV6Z. The newly optimized parameters can significantly improve the energy-predictive abilities of ten published formulas. Notably, all ten formulas perform quite similarly under the new platform with the reoptimized parameters. Finally, we built an online calculator for researchers to use for these extrapolation schemes. Our work would reignite the interest and applications of the underestimated formulas.

Planar Tetracoordinate Carbons in Allene-Type Structures.

The exhaustive exploration of the potential energy surfaces of CE2M2 (E = Si-Pb; M = Li and Na) revealed seven global minima containing a planar tetracoordinate carbon (ptC). The design, based on a π-localization strategy, resulted in a ptC with two double bonds forming a linear or a bent allene-type E═C═E motif. The magnetic response of the bent E═C═E fragments support a σ-aromaticity. The bonding analysis indicated that the ptCs form C-E covalent bonds and C-M electrostatic interactions.

All-nitrogen spiropentadiene-N5.

Of the pentanitrogen cation (N5 +) family, the only experimentally known isomer is the V-shaped structure 01. Here, we showed that a super-high-energy (∼100 kcal/mol above 01) all-nitrogen spiropentadiene 02 with considerable σ-delocalization deserves pursuit as the first spirocyclic all-nitrogen molecule, at least spectroscopical.

Mono-silicon isoelectronic replacement in CAl4 : van't hoff/le bel carbon or not?

In cluster studies, the isoelectronic replacement strategy has been successfully used to introduce new elements into a known structure while maintaining the desired topology. The well-known penta-atomic 18 valence electron (ve) species C Al 4 2 - and its Al- /Si or Al/Si+ isoelectronically replaced clusters CAl3 Si- , CAl2 Si2 , C AlSi 3 - , and C Si 4 2 + , all possess the same anti-van't Hoff/Le Bel skeletons, that is, nontraditional planar tetracoordinate carbon (ptC) structure. In this article, however, we found that such isoelectronic replacement between Si and Al does not work for the 16ve-CAl4 with the traditional van't Hoff/Le Bel tetrahedral carbon (thC) and its isoelectronic derivatives CAl3 X (X = Ga/In/Tl). At the level of CCSD(T)/def2-QZVP//B3LYP/def2-QZVP, none of the global minima of the 16ve mono-Si-containing clusters CAl2 SiX+ (X = Al/Ga/In/Tl) maintains thC as the parent CAl4 does. Instead, X = Al/Ga globally favors an unusual ptC structure that has one long C─X distance yet with significant bond index value, and X = In/Tl prefers the planar tricoordinate carbon. The frustrated formation of thC in these clusters is ascribed to the CSi bonding that prefers a planar fashion. Inclusion of chloride ion would further stabilize the ptC of CAl2 SiAl+ and CAl2 SiGa+ . The unexpectedly disclosed CAl2 SiAl+ and CAl2 SiGa+ represent the first type of 16ve-cationic ptCs with multiple bonds. © 2019 Wiley Periodicals, Inc.

A motif for heteronuclear C[triple bond, length as m-dash]E (E = Si, Ge, Sn, Pb) bonding: Lewis acid-base pair strategy.

The design and characterization of the heteronuclear group 14 C[triple bond, length as m-dash]E (E = Si, Ge, Sn, Pb) triple bonds have attracted intensive interest in the past few decades. In the current work, utilizing the advantages of N-heterocyclic carbenes (NHCs) and Lewis acid-base pair strategy, we theoretically designed a new class of compounds III-1, i.e., (NHCAR)C[triple bond, length as m-dash]E(Al(C6F5)3). Quantum chemical calculations showed that these singlet compounds possess very favourable isomerization, fragmentation and dimerization stabilities at the B3LYP/def2-TZVPP//B3LYP/def2-SVP level. The calculated bond lengths of CE in III-1 are 1.63 Å for Si, 1.70 Å for Ge, 1.91 Å for Sn and 2.01 Å for Pb, respectively, which are close to or even shorter than the known C[triple bond, length as m-dash]E bond lengths. In addition, the significant Mayer bond order values, two orthogonal π orbitals and one σ orbital between the C and E atoms also indicate the characteristics of triple bonds. Based on several bonding analyses, strong delocalization is found to exist between the C[triple bond, length as m-dash]E core and NHCAR forming a weak C[double bond, length as m-dash]C double bond. Hence, such obtained C[triple bond, length as m-dash]E species also can be described by their resonace structures as cunmulene analogs. In all, III-1 proposed here not only presents a universal C[triple bond, length as m-dash]E motif for all the heavier group 14 elements, but also provides a new strategy for the design and synthesis of heteronuclear group 14 triple bonds in the future.

A sixteen-valence-electron carbon-group 13 family with global penta-atomic planar tetracoordinate carbon: an ionic strategy.

The design of planar tetracoordinate carbon (ptC) has always been a challenge due to its unique bonding mode that necessitates the perfect balance between the carbon center and surrounding ligands both electronically and mechanically. A unique type of 18-valence-electron (18ve) template, i.e., CAl42-, has been found to be very effective in designing various novel 18ve-species upon skeletal substitution. In this work, we showed that though ptC is not the global structure for the parent 16ve-CAl4, suitable skeletal substitution can allow for a series of global minimum ptC species. Theoretical calculations at the level of CCSD(T)/def2-QZVP//B3LYP/def2-QZVP for 35 carbon-group 13 systems with 16-ve, i.e., CXaYbZcKd (X, Y, Z, K = Al/Ga/In/Tl; 0 ≤ a, b, c, d ≤ 4, a + b + c + d = 4), showed that 9 systems (CAl3Tl, CGa3Tl, CGa2Tl2, CAl2GaTl, CAl2InTl, CGa2InTl, CAlGa2Tl, CGa2InTl and CAlGaInTl) possess global minimum ptC and 2 systems (CAl3In and CAl2Tl2) have quasi-GM ptC. Except for CAl3Tl and CAl3In, all the ptCs were predicted for the first time. All these stable ptC structures have the same skeleton and can be described as the same ionic sub-structure, i.e., [A-]B+. This study not only enriches 16ve-ptC, but also directly demonstrates that utilizing an ionic strategy, non-ptC CAl4 also can be used as a template to extend the ptC family.

CAl3X (X = B/Al/Ga/In/Tl) with 16 valence electrons: can planar tetracoordinate carbon be stable?

As a perpetual chemical curiosity, planar tetracoordinate carbon (ptC) that violates the traditional tetrahedral carbon (thC) has made enormous achievements. In particular, the 18-valence-electron (18ve) counting rule has been found to be very effective in predicting ptC structures, as in CX42- (X = Al/Ga/In/Tl). By contrast, the corresponding neutral CX4 with 16ve each takes the thC form like methane. Herein, we report a mono-substituted neutral 16ve-CAl3X (X = Al/Ga/In/Tl). Our theoretical results showed that the competition between thC and ptC can be well tuned upon variation of X, and for X = In and Tl, the ptC structure becomes isoenergetic to and even more stable than thC, respectively. Thus, a low-lying ptC can be achieved in the 16ve-CAl3X set without acquiring additional electrons. This unintuitive result can be ascribed to the increased energetic preference of the ionic sub-structure [CAl3-]X+ from X = Al to Tl. We thus predict the first penta-atomic ptC species with 16ve, and the ionic strategy presented in this work is expected to promote novel designs of ptC molecules.

Theoretical Prediction on [5]Radialene Sandwich Complexes (CpM)2(C10H10) (Cp = η5-C5H5; M = Fe, Co, Ni): Geometry, Spin States, and Bonding.

[5]Radialene, the missing link for synthesis of radialene family, has been finally obtained via the preparation and decomplexation of the [5]radialene-bis-Fe(CO)3 complex. The stability of [5]radialene complex benefits from the coordination with Fe(CO)3 by losing free 1,3-butadiene structures to avoid polymerization. In light of the similar coordination ability of half-sandwiches CpM(Cp = η5-C5H5; M = Fe, Co, Ni), there is a great possibility that the sandwiched complexes of [5]radialene with CpM are available. Herein, we present the first theoretical prediction on the geometry, spin states and bonding of (CpM)(C10H10) and (CpM)2(C10H10). For M = Fe, Co, Ni, the ground states of (CpM)(C10H10) and (CpM)2(C10H10) are doublet and triplet, singlet and singlet, and doublet and triplet states, where each Fe, Co, and Ni adopts 17, 18, and 19 electron-configuration, respectively. In particular, (CpFe)2(C10H10) and (CpNi)2(C10H10) have considerable open-shell singlet features. Generally the trans isomers of (CpM)2(C10H10) with two CpM fragments on the opposite sides of the [5]radialene plane are apparently more stable than the cis ones with CpM fragments on the same side. However, for the singlet and triplet isomers of (CpNi)2(C10H10) (both cis and trans isomers), the energy differences are relatively small, indicating that these isomers all have the opportunity to exist. Besides, the easy Diels-Alder (DA) dimerization between the [3]dendralene-like fragments of (CpM)(C10H10) suggests the great difficulty in isolating the (CpM)(C10H10) monomer.

Isomerization Pathways of ONCNO: Unstable or Metastable?

Fulminates containing the CNO(-) ion have been widely utilized as high-energy density materials (HEDMs) for more than 120 years. Yet no purely covalently bound CNO molecule, i.e., nitrile oxide, is known to behave as an HEDM. In this study, we performed a thorough investigation of the potential energy surface of nitrile oxide ONCNO and related isomers, applying various sophisticated methods including G4, CBS-QB3, W1BD, CCSD(T)/CBS, and CASPT2/CBS. The Gibbs free energy calculations showed that the decomposition of ONCNO to the considerably endothermic products CNO + NO is favored compared to that into the highly exothermic products CO2 + N2. Thus, ONCNO fails to be the long expected nitrile oxide HEDM. However, with the rate-determining barrier of 23.3 kcal mol(-1) at the W1BD level, ONCNO should be experimentally accessible.

Transcriptome Profile Reveals that Pu-Erh Tea Represses the Expression of Vitellogenin Family to Reduce Fat Accumulation in Caenorhabditis elegans.

Due to misbalanced energy surplus and expenditure, obesity has become a common chronic disorder that is highly associated with many metabolic diseases. Pu-erh tea, a traditional Chinese beverage, has been believed to have numerous health benefits, such as anti-obesity. However, the underlying mechanisms of its anti-obesity effect are yet to be understood. Here, we take the advantages of transcriptional profile by RNA sequencing (RNA-Seq) to view the global gene expression of Pu-erh tea. The model organism Caenorhabditis elegans was treated with different concentrations of Pu-erh tea water extract (PTE, 0 g/mL, 0.025 g/mL, and 0.05 g/mL). Compared with the control, PTE indeed decreases lipid droplets size and fat accumulation. The high-throughput RNA-Sequence technique detected 18073 and 18105 genes expressed in 0.025 g/mL and 0.05 g/mL PTE treated groups, respectively. Interestingly, the expression of the vitellogenin family (vit-1, vit-2, vit-3, vit-4 and vit-5) was significantly decreased by PTE, which was validated by qPCR analysis. Furthermore, vit-1(ok2616), vit-3(ok2348) and vit-5(ok3239) mutants are insensitive to PTE triggered fat reduction. In conclusion, our transcriptional profile by RNA-Sequence suggests that Pu-erh tea lowers the fat accumulation primarily through repression of the expression of vit(vitellogenin) family, in addition to our previously reported (sterol regulatory element binding protein) SREBP-SCD (stearoyl-CoA desaturase) axis.

Unusually Short Be-Be Distances with and without a Bond in Be2 F2 and in the Molecular Discuses Be2 B8 and Be2 B7 (.).

Quantum-chemical calculations at the CCSD(T)/cc-pVTZ level of theory show that beryllium subfluoride, Be2 F2 , has a bond dissociation energy of De =76.9 kcal mol(-1) , which sets a record for the strongest Be-Be bond. The synthesis of this molecule should thus be possible in a low-temperature matrix. The discus-shaped species Be2 B8 and Be2 B7 (-) possess the shortest Be-Be distance for a molecule in the electronic ground state, but there is no Be-Be bond. The cyclic species Be2 B8 and Be2 B7 (-) exhibit double aromaticity with 6σ and 6π electrons, which strongly bind the Be2 fragment to the boron atoms. The very short interatomic distance between the beryllium atoms is due to the Be-B σ and π bonds, which operate like spokes in a wheel pressing the beryllium atoms together. The formation of the Be-B bonds has effectively removed the electronic charge of the valence space between the beryllium atoms. Along the Be-Be axis, there are two cage critical points adjacent to a ring critical point at the midpoint, but there is no bond critical point and no bond path.

Pentaatomic planar tetracoordinate silicon with 14 valence electrons: a large-scale global search of SiX(n)Y(m)(q) (n + m = 4; q = 0, ±1, -2; X, Y = main group elements from H to Br).

Designing and characterizing the compounds with exotic structures and bonding that seemingly contrast the traditional chemical rules are a never-ending goal. Although the silicon chemistry is dominated by the tetrahedral picture, many examples with the planar tetracoordinate-Si skeletons have been discovered, among which simple species usually contain the 17/18 valence electrons. In this work, we report hitherto the most extensive structural search for the pentaatomic ptSi with 14 valence electrons, that is, SiXnYm(q) (n + m = 4; q = 0, ±1, -2; X, Y = main group elements from H to Br). For 129 studied systems, 50 systems have the ptSi structure as the local minimum. Promisingly, nine systems, that is, Li3SiAs(2-), HSiY3 (Y = Al/Ga), Ca3SiAl(-), Mg4Si(2-), C2LiSi, Si3Y2 (Y = Li/Na/K), each have the global minimum ptSi. The former six systems represent the first prediction. Interestingly, in HSiY3 (Y = Al/Ga), the H-atom is only bonded to the ptSi-center via a localized 2c-2e σ bond. This sharply contradicts the known pentaatomic planar-centered systems, in which the ligands are actively involved in the ligand-ligand bonding besides being bonded to the planar center. Therefore, we proposed here that to generalize the 14e-ptSi, two strategies can be applied as (1) introducing the alkaline/alkaline-earth elements and (2) breaking the peripheral bonding. In light of the very limited global ptSi examples, the presently designed six systems with 14e are expected to enrich the exotic ptSi chemistry and welcome future laboratory confirmation.

How can carbon favor planar multi-coordination in boron-based clusters? Global structures of CB(x)E(y)(2-) (E = Al, Ga, x + y = 4).

With the high preference in forming multi-center bonding, boron has been a miracle ligand in constructing diverse planar multi-coordinate (pM) (tetra/hyper) species. Unfortunately, the boron ligand usually dislikes encompassing a pM carbon (pMC) due to the high competition with pM boron (pMB), which makes the realization of boron-based pMC very difficult and quite challenging. Herein, we propose a strategy that by means of cooperative doping and charge-compensation, we can successfully improve and tune the stability of pMC relative to pMB for CB4(2-). In the free CBxEy(2-) (E = Al/Ga) species, ptC is thermodynamically less stable than the global ptB in mono- and di-substituted systems, in agreement with the results of Boldyrev and Wang. However, the thermodynamic preference of pMC increases along with the Al/Ga-doping. The pMC species can be further stabilized by the introduction of the alkaline-earth counterion (Mg(2+)). CB2E2Mg (E = Al, Ga) designed in the present study represents the first successful design of a boron-based planar penta-coordinate carbon (ppC) structures as the global minima. The strategy proposed in this study should be useful in the manipulation of competition between exotic pMC and pMB in B-based systems.

Planar tetracoordinate carbons with a double bond in CAl3E clusters.

The potential energy surfaces of a series of clusters with the formula CAl3E (E = P, As, Sb, Bi) are systematically explored using density functional theory and high level ab initio calculations. The global minimum structure of these clusters contains a planar tetracoordinate carbon atom. The presence of a C=E double bond is supported by the Wiberg bond indices, the adaptive natural density partitioning analysis, and the magnetic response. Our results show that these planar tetracoordinate carbon clusters are both thermodynamically and kinetically viable species in the gas phase.

Silavinylidene stabilized by an N-heterocyclic carbene: a theoretically predicted stable molecule.

Quantum chemical calculations show that the N-heterocyclic carbene (NHC)-stabilized silavinylidene, NHC(tBu)→C=SiR2 is a strongly bonded complex, which has a linear arrangement of the donor and acceptor moieties. The molecule is the energetically lowest lying isomer of the NHC-stabilized R2CSi isomers and it is stable towards dimerization when R is a bulky substituent. The silavinylidene complex is the only species of the silylidene homologues NHC(tBu)→E=E'R2 (E, E' = C-Pb) which possesses a linear arrangement. The unusual bonding situation is explained in terms of donor-acceptor interactions between NHC(tBu) as σ donor and C=SiR2 in the doubly excited singlet state 3a1⇒2b2 which leads to a significantly shorter C-SiR2 bond compared with free C=SiR2 .

[Effects of acupuncture combined general anesthesia on endorphin and hemodynamics of laparoscopic cholecystectomy patients in the perioperative phase].

OBJECTIVE: To observe the effects of different anesthesia ways on endorphin and hemodynamics of laparoscopic cholecystectomy patients in the perioperative phase. METHODS: A total of 90 laparoscopic cholecystectomy patients, 29 to 80 years old, were randomly assigned to Group A (treated with electroacupuncture at acupoints combined general anesthesia), Group B (treated with electroacupuncture at non-acupoints combined general anesthesia), and Group C (treated with general anesthesia) according to American Society of Anesthesiologists (ASA) I-II, 30 cases in each group. All patients were induced by 3 microg/kg Fentanyl (Fen), 2 mg/kg Propofol (Pro), and 0.1 mg/kg Vecuronium (Vcr). Bispectral index (BIS), being 40 -65, indicated the state of general anesthesia. The anesthesia was maintained by intravenous injecting Pro, interruptedly intravenous injecting Fen and Vcr. Each patient received patient controlled intravenous analgesia (PCIA) after operation. On these bases, patients in Group A received electrical acupuncture at bilateral Hegu (LI4), Neiguan (PC6), Quchi (Ll11), Zusanli (ST36), and Yanglingquan (GB34). Patients in Group B received electrical acupuncture at the points beside acupoints. The electroacupuncture was lasted from 15 -30 min before anesthesia induction to the end of the operation in Group A and B. The heart rate (HR), mean arterial pressure (MAP), cardiac index (CI), cardiac output (CO), systemic vascular resistance index (SVRI), and acceleration index (ACI) were recorded before anesthesia induction, immediate before pneumoperitoneum, 5 min after pneumoperitoneum, excision of gallbladder, and at the end of operation. The time consumption from discontinuation to spontaneously breathing recovery, analeptic, and extubation were recorded. The blood samples (3 mL each time) were collected from the peripheral vein before anesthesia induction, 2 h after operation, the 1st day after operation, and the 3rd day after operation to detect the beta-endorphin (beta-EP) level. The visual analogue scale (VAS) were observed and recorded in the 3 groups at post-operative 4, 6, 8, 24, and 44 h, respectively. RESULTS: (1) Compared with before anesthesia induction in the same group, the CI, CO, ACI of all patients decreased significantly at 5 min after pneumoperitoneum and at excision of gallbladder (P < 0.01, P < 0.05). The HR, MAP, SVRI obviously increased in Group B and Group C at each time point (P < 0.05, P < 0.01). Less change happened in Group A. Compared with Group C, the increment of MAP was less in Group A at 5 min after pneumoperitoneum, showing statistical difference (P < 0.05). (2) The time consumption from discontinuation to analeptic and extubation was obviously shorter in Group A than in Group B and Group C (P < 0.05, P < 0.01). (3) The level of beta-EP on the 1st day of operation was significantly lower in Group A than in Group B (P < 0.05) and Group C (P < 0.01). (4) The VAS score at post-operative 44 h was significantly lower in Group A than in Group B and Group C (P < 0.05). CONCLUSIONS: Electroacupuncture at acupoints combined general anesthesia could maintain the stabilization of haemodynamics, and relieve the stress reaction after pneumoperitoneum and operation, and prolong it to early post-operative period, and strengthen the effects of post-operative analgesia. The post-operative recovery was fast, safe, and reliable.

Role of hydrocarbon radicals CH(x) (x=1, 2, 3) in graphene growth: a theoretical perspective.

Many outstanding properties of graphene are blocked by the existence of structural defects. Herein, we propose an important healing mechanism for the growth of graphene, which is produced via plasma-enhanced chemical vapor decomposition (PECVD), that is, the healing of graphene with single vacancies by decomposed CH(4) (hydrocarbon radical CH(x), x=1, 2, 3). The healing processes undergo three evolutionary steps: 1) the chemisorption of the hydrocarbon radicals, 2) the incorporation of the C atom of the hydrocarbon radicals into the defective graphene, accompanied by the adsorption of the leaving H atom on the graphene surface, 3) the removal of the adsorbed H atom and H(2) molecule to generate the perfect graphene. The overall healing processes are barrierless, with a huge released heat of 530.79, 290.67, and 159.04 kcal mol(-1), respectively, indicative of the easy healing of graphene with single vacancies by hydrocarbon radicals. Therefore, the good performance of the PECVD method for the generation of graphene might be ascribed to the dual role of the CH(x) (x=1, 2, 3) species, acting both as carbon source and as defect healer.

Free and Complexed Fluoropentazoles: Anomalous Ring Charge and Appreciable Kinetic Stability.

Fluoronitrogens are strongly related to high-energy density materials. Fluoropentazole (cyclo-FN5) with hitherto the highest N/F ratio at ambient pressure belongs to the family of well-known and 119-year-old pentazoles. Due to the presence of the overwhelmingly electronegative F element, cyclo-FN5 is the only pentazole to date that contains a cationic N5 ring. However, also due to F's large electronegativity, the reported ring-destruction barrier of cyclo-FN5 is nearly the lowest (6.7 kcal/mol), which has made its synthesis and characterization quite difficult. Here, cyclo-FN5 was re-predicted to bear an almost doubled ring-destruction barrier (∼12-14 kcal/mol) at the CBS-QB3, G4, W1BD, CCSD(T)/CBS//CCSD/cc-pVTZ, and CCSD(T)/CBS//CCSD(T)/aug-cc-pVTZ levels. Promisingly, upon further complexation with Lewis acid(s), the ring-destruction barrier of cyclo-FN5 might reach 23 kcal/mol, which is comparable to that of the well-known and already synthesized arylpentazoles (∼20 kcal/mol), and the positive charge on the N5 ring can be increased to 0.66 |e|.

Planar tetracoordinate carbon versus planar tetracoordinate boron: the case of CB4 and its cation.

In this study, we analyzed CB(4) and its cation, CB(4)(+). Using CCSD(T)/aug-cc-pVQZ//CCSD(T)/aug-cc-pVTZ quantum-chemical calculations, we found that the neutral molecule is in accord with the results of Boldyrev and Wang, having a C(s) global minimum with a planar tricoordinate carbon structure, contradicting previous studies. In contrast, CB(4)(+), which was reported by an early mass spectroscopic study, has a planar tetracoordinate carbon (ptC) atom, demonstrating that a modification of the charge can promote the stabilization of a ptC structure.