Ambident Nucleophilic Substitution: Understanding Non-HSAB Behavior through Activation Strain and Conceptual DFT Analyses

The ability to understand and predict ambident reactivity is key to the rational design of organic syntheses. An approach to understand trends in ambident reactivity is the hard and soft acids and bases (HSAB) principle. The recent controversy over the general validity of this principle prompted us to investigate the competing gas-phase S_N2 reaction channels of archetypal ambident nucleophiles CN⁻, OCN⁻, and SCN⁻ with CH₃Cl (S_N2@C) and SiH₃Cl (S_N2@Si), using DFT calculations. Our combined analyses highlight the inability of the HSAB principle to correctly predict the reactivity trends of these simple, model reactions. Instead, we have successfully traced reactivity trends to the canonical orbital-interaction mechanism and the resulting nucleophile–substrate interaction energy. The HOMO–LUMO orbital interactions set the trend in both S_N2@C and S_N2@Si reactions. We provide simple rules for predicting the ambident reactivity of nucleophiles based on our Kohn–Sham molecular orbital analysis.     

Magnesium Halide‐promoted Ring‐opening Reaction of Cyclic Ether in the Presence of Phosphine Halide

A new route to the direct preparation of H‐phosphinate esters has been explored. The ring‐opening reaction of cyclic ether (tetrahydrofuran or tetrahydropyrane) was carried out with magnesium halide in the presence of phosphine halide (PRCl₂ or PCl₃). The process is straightforward and all the reagents are relatively cheap and readily available. Magnesium halide‐mediated THF ring‐opening (S_N2@C) and the subsequent S_N2@P elementary reactions that giving rise to the intermediate of haloalkyl phosphinates have been discussed based on our experimental findings ( Path I : S_N2@C−+S_N2@P). Another possible route, the direct S_N2 between THF (nucleophile) and phosphine halide (electrophile) that followed by THF ring opening by halide dissociated from phosphine halide ( Path II: S_N2@P−+S_N2@C), was also proposed. However, path II is the least likely reaction path because neutral THF is not a good nucleophile. H‐phosphinate esters could be readily available in the subsequent hydrolysis process. Considering the ionic bond strength in magnesium halides and the nucleophilicity of halides dissociated from MgX₂ in protic solvents like water, MgBr₂ is recommended for ring‐opening reactions of cyclic ethers.     

Density functional theory study on the reactions of X− with CH3SY (X,Y = F,Cl, Br,I)

Gas‐phase anionic reactions X^? + CH₃SY (X, Y = F, Cl, Br, I) have been investigated at the level of B3LYP/6‐311+G (2df,p). Results show that the potential energy surface (PES) of gas‐phase reactions X^? + CH₃SY (X, Y = Cl, Br, I) has a quadruple‐well structure, indicating an addition–elimination (A–E) pathway. The fluorine behaves differently in many respects from the other halogens and the reactions F^? + CH₃SY (Y = F, Cl, Br, I) correspond to deprotonation instead of substitution. The gas‐phase reactions X^? + CH₃SF (X = Cl, Br, I), however, follow an A–E pathway other than the last two out going steps (COM2 and PR) that proceeds via a deprotonation. The polarizable continuum model (PCM) has been used to evaluate the solvent effects on the energetics of the reactions X^? + CH₃SY (X, Y = Cl, Br, I). The PES is predicted to be unimodal in the solvents of high polarity. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Theoretical investigation on identical anionic halide‐exchange SN2 reaction processes on N‐haloammonium cation NH3X+ (X = F,Cl, Br,and I)

CCSD(T) calculations have been used for identically nucleophilic substitution reactions on N‐haloammonium cation, X^? + NH₃X⁺ (X = F, Cl, Br, and I), with comparison of classic anionic S_N2 reactions, X^? + CH₃X. The described S_N2 reactions are characterized to a double curve potential, and separated charged reactants proceed to form transition state through a stronger complexation and a charge neutralization process. For title reactions X^? + NH₃X⁺, charge distributions, geometries, energy barriers, and their correlations have been investigated. Central barriers ΔE for X^? + NH₃X⁺ are found to be lower and lie within a relatively narrow range, decreasing in the following order: Cl (21.1 kJ/mol) > F (19.7 kJ/mol) > Br (10.9 kJ/mol) > I (9.1 kJ/mol). The overall barriers ΔE relative to the reactants are negative for all halogens: ?626.0 kJ/mol (F), ?494.1 kJ/mol (Cl), ?484.9 kJ/mol (Br), and ?458.5 kJ/mol (I). Stability energies of the ion–ion complexes ΔE_comp decrease in the order F (645.6 kJ/mol) > Cl (515.2 kJ/mol) > Br (495.8 kJ/mol) > I (467.6 kJ/mol), and are found to correlate well with halogen Mulliken electronegativities (R² = 0.972) and proton affinity of halogen anions X^? (R² = 0.996). Based on polarizable continuum model, solvent effects have investigated, which indicates solvents, especially polar and protic solvents lower the complexation energy dramatically, due to dually solvated reactant ions, and even character of double well potential in reactions X^? + CH₃X has disappeared. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

The α‐effect exhibited in gas‐phase SN2@N and SN2@C reactions

In order to explore the existence of α‐effect in gas‐phase S_N2@N reactions, and to compare its similarity and difference with its counterpart in S_N2@C reactions, we have carried out a theoretical study on the reactivity of six α‐oxy‐Nus (FO^?, ClO^?, BrO^?, HOO^?, HSO^?, H₂NO^?) in the S_N2 reactions toward NR₂Cl (R = H, Me) and RCl (R = Me, i‐Pr) using the G2(+)_M theory. An enhanced reactivity induced by the α‐atom is found in all examined systems. The magnitude of the α‐effect in the reactions of NR₂Cl (R = H, Me) is generally smaller than that in the corresponding S_N2 reaction, but their variation trend with the identity of α‐atom is very similar. The origin of the α‐effect of the S_N2@N reactions is discussed in terms of activation strain analysis and thermodynamic analysis, indicating that the α‐effect in the S_N2@N reactions largely arises from transition state stabilization, and the “hyper‐reactivity” of these α‐Nus is also accompanied by an enhanced thermodynamic stability of products from the n_(N) → σ*_(O?Y) negative hyperconjugation. Meanwhile, it is found that the reactivity of oxy‐Nus in the S_N2 reactions toward NMe₂Cl is lower than toward i‐PrCl, which is different from previous experiments, that is, the S_N2 reactions of NH₂Cl is more facile than MeCl. © 2013 Wiley Periodicals, Inc.     

Counter-ion effect in the nucleophilic substitution reactions at silicon: a G2M(+) level theoretical investigation

Three identity nucleophilic substitution reactions at tetracoordinated silicon atom with inversion and retention pathways: Nu + SiH₃Cl → Nu + SiH₃Cl[Nu = (1)Cl⁻, (2) LiCl, and (3) (LiCl)₂], are investigated using the G2M(+) theory. Our results show that changing the nucleophile can shift the mechanism (favorable pathway), stepwise from a single-well PES for reaction 1, via a double-well PES for reaction 2, to a triple-well PES for reaction 3, indicating the importance of steric and electronic effects on the S_N2@Si. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Nucleophilic substitution at silicon (SN2@Si) via a central reaction barrier

It is textbook knowledge that nucleophilic substitution at carbon (SN2@C) proceeds via a central reaction barrier which disappears in the corresponding nucleophilic substitution reaction at silicon (SN2@Si). Here, we address the question why the central barrier disappears from SN2@C to SN2@Si despite the fact that these processes are isostructural and isoelectronic. To this end, we have explored and analyzed the potential energy surfaces (PES) of various Cl-+CR3Cl (R=H, CH3) and Cl-+SiR3Cl model reactions (R=H, CH3, C2H5, and OCH3). Our results show that the nature of the SN2 reaction barrier is in essence steric, but that it can be modulated by electronic factors. Thus, simply by increasing the steric demand of the substituents R around the silicon atom, the SN2@Si mechanism changes from its regular single-well PES (with a stable intermediate transition complex, TC), via a triple-well PES (with a pre- and a post-TS before and after the central TC), to a double-well PES (with a TS; R=OCH3), which is normally encountered for SN2@C reactions.     

Theoretical Study of Nucleophilic Identity Substitution Reactions at Nitrogen,Silicon and Phosphorus versus Carbon: Reaction Pathways,Energy Barrier,Inversion and Retention Mechanisms

Gas‐phase identity S_N2(N), S_N2(Si) and S_N2(P) versus S_N2(C) reactions with Cl^? are investigated by the ab initio method. Front‐side attack identity S_N2 reactions considered have all double‐well potential energy surfaces (PES), and back‐side attack identity S_N2(C) and S_N2(N) reactions have also double‐well PES, while back‐side attack identity S_N2(Si) and S_N2(P) have single‐well PES. In addition, the geometrical transformations, potential energy profiles of front‐side and back‐side attack identity S_N2(N), S_N2(Si) and S_N2(P) versus S_N2(C) reactions based on the IRC calculations are described, the differences between them for the front‐side or back‐side attack reactions have been demonstrated.     

Kinetics and mechanism of some fast anation reactions of a series of substituted dien complexes of palladium(II). Temperature and pressure dependencies in weakly acidic aqueous solution1

Anation reactions of the type [Pd(L)(H₂O)]²⁺ + X^? »[Pd(L)X]⁺ + H₂O with L = 1, 4, 7-Et₃dien, 1, 1, 7, 7-Me₄dien and 1, 1, 4, 7, 7-Me₅dien and X^? = Cl^?, Br^?, I^? and N₃? have been studied kinetically as a function of [X?], temperature and pressure (up to 1 kbar). Second-order anation rate constants decrease with an increase in the size of L, and are accompanied by an increase in ΔH≠. For a given L the sequence Cl^? < Br^? < I^? < N₃^? holds, and the values of ΔS≠ and ΔV≠ are consistent with an associative mechanism. The results are discussed with reference to similar anation reactions previously investigated.     

Frontside versus Backside SN2 Substitution at Group 14 Atoms: Origin of Reaction Barriers and Reasons for Their Absence

We have theoretically studied the gas‐phase nucleophilic substitution at group‐14 atoms (S_N2@A) in the model reactions of Cl^?+AH₃Cl (A=C, Si, Ge, Sn, and Pb) using relativistic density functional theory (DFT) at ZORA‐OLYP/TZ2P. Firstly, we wish to explore and understand how the reaction coordinate ζ, and potential energy surfaces (PES) along ζ, vary as the center of nucleophilic attack changes from carbon to the heavier group‐14 atoms. Secondly, a comparison between the more common backside reaction (S_N2‐b) and the frontside pathway (S_N2‐f) is performed. The S_N2‐b reaction is found to have a central barrier for A=C, but none for the other group‐14 atoms, A=Si–Pb. Relativistic effects destabilize reactant complexes and transition species by up to 10 kcal mol^?1 (for S_N2‐f@Pb), but they do not change relative heights of barriers. We also address the nature of the transformation in the frontside S_N2‐f reactions in terms of turnstile rotation versus Berry‐pseudorotation mechanism.     

Interpretation of collision induced dissociative charge-transfer processes in rare-gas molecule ions

We propose an interpretation of experimental measurements of dissociative charge-transfer processes X₂⁺+Y→X+X+Y⁺ (X=rare-gas atom) in terms of avoided-crossings of adiabatic potential surfaces. Model potential energy surfaces for a typical system (X=He, Y=Ne) are computed by the method of diatomics-in-molecules (DIM). The qualitative shapes of the surfaces suggest dynamical simplifications which can be embodied in a classical-mechanical trajectory model with “surface-hopping”. Analogy with earlier surface-hopping trajectory calculations and with trajectories for endothermic ion-molecule reactons provides a basis for understanding some of the major experimental findings for the He₂⁺-Ne reaction. The model viewpoint is also able to rationalize the observance (or non-observance) of other rare-gas reactions and can be extended to the case where Y=N₂, X=He.     

An electron localization function and catastrophe theory analysis on the molecular mechanism of gas-phase identity SN2 reactions

A set of four reactions, XCH₃+X^? (X=F, Cl, Br) and ClSiH₃+Cl^?, is investigated by means of the joint use of the electron localization function (ELF) and catastrophe theory (CT) analysis in order to obtain new insights into the bond breaking/forming processes for identity S_N2 gas-phase reactions. Using DFT calculations at the OLYP/6-311++G(d,p) level, the effect of nucleophile (F, Cl, and Br anions) and the role of reacting centers (C or Si) on the reaction mechanisms are investigated. The charge-shift character of carbon–halogen bonds is studied by determination of the weights of the Lewis resonance structures. In all S_N2 reactions at the carbon atom, there is a progressive reduction on the covalent character of the C–X bond from the reactant complex (0.41, 0.57, 0.58 for F, Cl, and Br, respectively) until the bond-breaking process, occurring before the transition structure is reached. On the other hand, the Si–Cl bond maintains its degree of covalent character (0.51) from the isolated fragments to the formation of a stable transition complex, presenting two silicon–chlorine charge-shifted bonds. The analysis of the ELF topology along the reaction path reveals that all reactions proceed via the same turning points of fold-type but the order is inverted for reactions taking place at C or Si atoms.     

A computational study on [(PH<Subscript>2</Subscript>X)<Subscript>2</Subscript>]<Superscript>·+</Superscript> homodimers involving intermolecular two-center three-electron bonds

A computational study at CCSD(T) theoretical level has been carried out on radical cation [(PH₂X)₂]^·+ homodimers. Four stable minima configurations have been found for seven substituted phosphine derivatives, X = H, CH₃, CCH, NC, OH, F and Cl. The most stable minimum presents an intermolecular two-center three-electron P···P bond except for X = CCH. The other three minima correspond to an alternative P···P pnicogen bonded complex, to a P···X contact and the last one to the complex resulting from a proton transfer, PH₃X⁺:PHX^·. The complexes obtained have been compared with those of the corresponding neutral ones, (PH₂X)₂, and the analogous protonated ones, PH₃X⁺:PH₂X, recently described in the literature. The spin and charge densities of the complexes have been examined. The electronic characteristics of the complexes have been analyzed with the NBO and AIM methods. The results obtained for the spin density, charge and NBO are coherent for all the complexes.     

Distonic radical cations : Guidelines for the assessment of their stability

Ab initio molecular orbital calculations on the distonic radical cations CH₂(CH₂)_nN⁺H₃ and their conventional isomers CH₃(CH₂)_nNH₂+ (n = 0,1, 2 and 3) indicate a preference in each case for the distonic isomer. The energy difference appears to converge with increasing n towards a limit which is close to the energy difference between the component systems CH₃·H₂+CH₃^+NH₃ (representing the distonic isomer) and CH₃CH₃+CH₃NH₂+ (representing the conventional isomer). The generality of this result is assessed by using results for the component systems CH₃·Y+CH₃X⁺H and CH₃YH+CH₃X^+. (or CH₃YH^+. + CH₃X) to predict the relative energies of the distonic ions ·Y(CH₂)_nX⁺H and their conventional isomers HY(CH₂)nX^+. (X = NH₂, OH, F, PH₂, SH, Cl; Y = CH₂, NH, O) and testing the predictions through explicit calculations for systems with n = 0,1 and 2. Although the predictions based on component systems are often close to the results of direct calculations, there are substantial discrepancies in a number of cases; the reasons for such discrepancies are discussed. Caution must be exercised in applying this and related predictive schemes. For the systems examined in the present study, the conventional radical cation is predicted in most cases to lie lower in energy than its distonic isomer. It is found that the more important factors contributing to a preference for distonic over conventional radical cations are the presence in the system of a group(X) with high proton affinity and the absence of a group (X, Y or perturbed (C—C) with low ionization energy.     

Computational study of SN2 reactions promoted by crown ether: Contact ion pair versus solvent‐separated ion pair mechanism

We examined by quantum chemical methods the mechanism of S_N2 reaction using metal bromide MBr (M = Na, K, Cs) and KX (X= F, Cl) in CH₃CN promoted by crown ether (18‐crown‐6). We focus on whether the metal salts react as a contact ion pair (CIP; M⁺ and X^– in close contact) or as a solvent‐separated ion pair (SSIP; M⁺ and X^– at large distance). In SSIP mechanism, X^– is removed far enough from M⁺ for the metal salt to be considered as “separated” by the effects of the crown ether and the solvent. In the CIP picture, conversely, the coordination of 18‐crown‐6 to M⁺ is not sufficient to overcome the powerful Coulombic interactions between M⁺ and X^–. We find that the CIP route is favored for S_N2 bromination processes using MBr (M = Na, K, Cs). For S_N2 reaction using KF, the feasibility of the two pathways is essentially equal, whereas for S_N2 chlorination by KCl the SSIP route is predicted to be favored.     

Nucleophilic substitution at phosphorus: stereochemistry and mechanisms

This review is devoted to the stereochemistry of nucleophilic substitution reactions at phosphorus. The study of the reactions of phosphoryl group transfer is important for biological and molecular chemistry. The stereochemistry and mechanisms of S_N1(P) monomolecular and S_N2(P) bimolecular nucleophilic substitution reactions of organophosphorus compounds are discussed. It has been shown that hydrolysis of many natural phosphates proceeds according to the monomolecular S_N1(P) mechanism via the formation of metaphosphate intermediate (PO₃^?). S_N2(P) nucleophilic substitution at chiral trivalent or pentavalent phosphorus compounds proceeds via the formation of penta-coordinated transition state or pentacoordinate intermediate.     

A Unified Framework for Understanding Nucleophilicity and Protophilicity in the SN2/E2 Competition

The concepts of nucleophilicity and protophilicity are fundamental and ubiquitous in chemistry. A case in point is bimolecular nucleophilic substitution (S_N2) and base-induced elimination (E2). A Lewis base acting as a strong nucleophile is needed for S_N2 reactions, whereas a Lewis base acting as a strong protophile (i.e., base) is required for E2 reactions. A complicating factor is, however, the fact that a good nucleophile is often a strong protophile. Nevertheless, a sound, physical model that explains, in a transparent manner, when an electron-rich Lewis base acts as a protophile or a nucleophile, which is not just phenomenological, is currently lacking in the literature. To address this fundamental question, the potential energy surfaces of the S_N2 and E2 reactions of X⁻+C₂H₅Y model systems with X, Y = F, Cl, Br, I, and At, are explored by using relativistic density functional theory at ZORA-OLYP/TZ2P. These explorations have yielded a consistent overview of reactivity trends over a wide range in reactivity and pathways. Activation strain analyses of these reactions reveal the factors that determine the shape of the potential energy surfaces and hence govern the propensity of the Lewis base to act as a nucleophile or protophile. The concepts of “characteristic distortivity” and “transition state acidity” of a reaction are introduced, which have the potential to enable chemists to better understand and design reactions for synthesis.     

Dynamical Bifurcation in Gas‐Phase XH− + CH3Y SN2 Reactions: The Role of Energy Flow and Redistribution in Avoiding the Minimum Energy Path

The gas‐phase reactions of XH^? (X=O, S) + CH₃Y (Y=F, Cl, Br) span nearly the whole range of S_N2 pathways, and show an intrinsic reaction coordinate (IRC) (minimum energy path) with a deep well owing to the CH₃XH???Y^? (or CH₃S^????HF) hydrogen‐bonded postreaction complex. MP2 quasiclassical‐type direct dynamics starting at the [HX???CH₃???Y]^? transition‐state (TS) structure reveal distinct mechanistic behaviors. Trajectories that yield the separated CH₃XH+Y^? (or CH₃S^?+HF) products directly are non‐IRC, whereas those that sample the CH₃XH???Y^? (or CH₃S^????HF) complex are IRC. The IRCIRC/non‐IRC ratios of 90:10, 40:60, 25:75, 2:98, 0:100, and 0:100 are obtained for (X, Y)=(S, F), (O, F), (S, Cl), (S, Br), (O, Cl), and (O, Br), respectively. The properties of the energy profiles after the TS cannot provide a rationalization of these results. Analysis of the energy flow in dynamics shows that the trajectories cross a dynamical bifurcation, and that the inability to follow the minimum energy path arises from long vibration periods of the X?C???Y bending mode. The partition of the available energy to the products into vibrational, rotational, and translational energies reveals that if the vibrational contribution is more than 80 %, non‐IRC behavior dominates, unless the relative fraction of the rotational and translational components is similar, in which case a richer dynamical mechanism is shown, with an IRC/non‐IRC ratio that correlates to this relative fraction.     

Collision-Induced Decomposition (CID) of Triangular [Mo3S7-xSex]4+ Complexes (x = 0,3,7). A liquid SIMS and FTMS/MS study of [Mo3S7-xSex(Et2NCS2)3] +

[Mo₃S(S₂)₃(dtc)₃]I, [Mo₃S(SeS)₃(dtc)₃](dtc), and [Mo₃Se(Se₂)₃(dtc)₃](dtc) (dtc = N,N-diethyldithiocarbamate) were investigated by liquid SIMS-FTMS. The fragmentation pathways were essentially the same for the three compounds and can be explained by two types of fragmentation processes: stepwise abstraction of S/Se atoms as exemplified by the series [Mo₃X_z(dtc)₃]⁺ (4 ? z ? 7, X = S, Se), and ligand dissociation, as indicated by the generation of [Mo₃X_z(dtc)₂]⁺ (5 ? z ? 7, X = S, Se). The exclusive elimination of the Se-atoms from [Mo₃S(S_ax-Se_eq)₃(dtc)₃]⁺ confirmed the inequivalent reactivity of the bridging atoms in axial and equatorial position as observed in previous studies. Collision-induced decomposition (CID) of [Mo₃S₇(dtc)₃]⁺ ( 1 ), [Mo₃S₆(dtc)₃]⁺ ( 2 ), [Mo₃S(S_ax–Se_eq)₃(dtc)₃]⁺ ( 3 ), and [Mo₃Se₇(dtc)₃]⁺ ( 4 ) revealed distinctly different fragmentation reactions for the SIMS and CID mode. CID of 1, 3 , and 4 resulted in a two-step reaction with the exclusive elimination of diatomic molecules XY (X,Y = S/Se). In the case of 3 , the selective elimination of Se₂ indicated the abstraction of two Se-atoms located in equatorial positions of two different bridging groups. This result is discussed in terms of mechanisms, based on labile M? X_eq and inert M? X_ax bonds with an intramolecular formation of a X₄ fragment prior to the elimination of X₂.     

Impact of the anion and chalcogen on the crystal structure and properties of 4,6‐dimethyl‐2‐pyrimido(thio)nium halides

By the reaction of urea or thiourea, acetylacetone and hydrogen halide (HF, HBr or HI), we have obtained seven new 4,6‐dimethyl‐2‐pyrimido(thio)nium salts, which were characterized by single‐crystal X‐ray diffraction, namely, 4,6‐dimethyl‐2‐oxo‐2,3‐dihydropyrimidin‐1‐ium bifluoride, C₆H₉N₂O⁺·HF₂^? or (dmpH)F₂H, 4,6‐dimethyl‐2‐oxo‐2,3‐dihydropyrimidin‐1‐ium bromide, C₆H₉N₂O⁺·Br^? or (dmpH)Br, 4,6‐dimethyl‐2‐oxo‐2,3‐dihydropyrimidin‐1‐ium iodide, C₆H₉N₂O⁺·I^? or (dmpH)I, 4,6‐dimethyl‐2‐oxo‐2,3‐dihydropyrimidin‐1‐ium iodide–urea (1/1), C₆H₉N₂O⁺·I^?·CH₄N₂O or (dmpH)I·ur, 4,6‐dimethyl‐2‐sulfanylidene‐2,3‐dihydropyrimidin‐1‐ium bifluoride–thiourea (1/1), C₆H₉N₂S⁺·HF₂^?·CH₄N₂S or (dmptH)F₂H·tu, 4,6‐dimethyl‐2‐sulfanylidene‐2,3‐dihydropyrimidin‐1‐ium bromide, C₆H₉N₂S⁺·Br^? or (dmptH)Br, and 4,6‐dimethyl‐2‐sulfanylidene‐2,3‐dihydropyrimidin‐1‐ium iodide, C₆H₉N₂S⁺·I^? or (dmptH)I. Three HCl derivatives were described previously in the literature, namely, 4,6‐dimethyl‐2‐oxo‐2,3‐dihydropyrimidin‐1‐ium chloride, (dmpH)Cl, 4,6‐dimethyl‐2‐sulfanylidene‐2,3‐dihydropyrimidin‐1‐ium chloride monohydrate, (dmptH)Cl·H₂O, and 4,6‐dimethyl‐2‐sulfanylidene‐2,3‐dihydropyrimidin‐1‐ium chloride–thiourea (1/1), (dmptH)Cl·tu. Structural analysis shows that in 9 out of 10 of these compounds, the ions form one‐dimensional chains or ribbons stabilized by hydrogen bonds. Only in one compound are parallel planes present. In all the structures, there are charge‐assisted N⁺—H…X^? hydrogen bonds, as well as weaker C_Ar⁺—H…X^? and π⁺…X^? interactions. The structures can be divided into five types according to their hydrogen‐bond patterns. All the compounds undergo thermal decomposition at relatively high temperatures (150–300 °C) without melting. Four oxopyrimidinium salts containing a π⁺…X^?…π⁺ sandwich‐like structural motif exhibit luminescent properties.