Effects of internal hydrogen bonds between amide groups: Gas-phase basicity and proton affinity of linear aliphatic dicarboxamides

The apparent gas-phase basicity GB(app) and the proton affinity PA of aliph atic dicarboxamides has been determined by tandem mass spectrometry using t he kinetic method. The diamides analyzed are primary maleic acid diamide (l a), fumaric acid diamide (2a) and trans,trans-muconic acid diamide (3a) and the tertiary N,N,N:N'-tetramethyl derivatives 2b and 3b, which have an alm ost fixed relative orientation of the terminal amide groups because of the C-C double bond(s) in the carbon skeleton, and the 1,n-dicarboxamides of su ccinic acid (4a) and (4b), of glutaric acid (5a) and (5b), of adipinic acid (6a) and (6b) and of sebacinic acid (7a) and (7b) containing a flexible -( CH2)(a) chain with n = 2, 3, 4 and 8. Very large differences are observed f or GB(app) derived from the dissociation of proton-bound heterodimers eithe r as metastable ions or by CID for all diamides which are expected to form an internal proton bridge between the carbonyl-O atoms of the terminal amid e groups in the protonated species. These effects indicate considerable con formational changes of the diamides by protonation and entropic effects acc ompanying the dissociation of their proton-bound heterodimers, To study the effect of collisional activation, which is believed to alter the effective temperature, T-eff, of the proton-bound dimer ions, on their dissociation, separate experiments have been performed with thermalized proton bound het erodimers of 5a using Fourier transform ion cyclotron resonance (FT-ICR) sp ectrometry to control the collision energy. The evaluation of T-eff from th ese experiments and the use of T-eff in Van't Hoff plots to determine the P A of 5a shows a surprisingly good agreement with the results from tandem ma ss spectrometry, which supports the view that the kinetic method using diff erent T-eff can be used to determine the PA and the difference of the "appa rent entropy" of protonation, Delta(DeltaS(H+))(app), of the compound under study and the reference base of the proton bound heterodimer from GB(app) even in the case of large entropy effects. The PA of maleic acid diamide la and its trans isomer 2a, not building an internal proton bridge by protona tion, differ by 80 kJ mol(-1). A value of -19 J mol(-1) K is obtained for ( DeltaS(H+))(app) (la) while entropic effects are essentially absent in the case of 2a, The PA of linear dicarboxamides 4a-7a increases with the length of the -(CH2)(n) chain and exceeds that of monoamides of a comparable size by 60-100 kJ mol(-1). This is attributed to the formation of an internal p roton bridge and a release of constraints for the internal proton bridge fo r the longer chains. Delta(DeltaS(H+))(app) is practically constant for 5a- 7a at a value of -41 +/- 2 J mol(-1) K, but only -19 J mol(-1) K for 4a, Th is can be understood if the entropy loss during protonation of the diamides is mostly due to loss of internal rotations of the amide groups. In contra st to the primary amides, the tertiary dicarboxamides 4b-7b display identic al PA independent of the length of the -(CH2)(n) chain, and the effect of f ormation of the internal proton bridge on the PA is distinctly less than fo r primary dicarboxamides, In addition, a constant value of only -16 +/- 3 J mol(-1) K is obtained for Delta(DeltaS(H+))(app) of 4b-7b. These results a re interpreted by different types of the proton bridges of primary and tert iary diamides. Primary linear dicarboxamides generate a true proton bridge between the car bonyl-O atoms of the terminal amide groups as corroborated by ab initio cal culations of their structures. In contrast, protonated tertiary dicarboxami des display properties of (internal) ion/dipole complexes in which the prot onated amide group is "solvated" by the second one.