Chemical structure of the HPTS molecule (left) and of the 2,4,6 tricyanophenol (TCN, right; see text). Both molecules are functionalized phenols, i.e., have an OH group linked to an aromatic ring. Ground-state HPTS is a weak acid much like unsubstituted phenol , but its first singlet excited-state as well as ground-state 2,4,6 tricyanophenol are moderately strong acids, . Also shown is the numbering of the ortho (2 and 6) and para (4) positions.
A graphical illustration of the present ground-state, dipole field/QM approach. (a) An exact representation of the experimental system should involve a first-principle description of excited-state HPTS, ground-state solvent, and proton acceptor molecules. X should be either a methyl, or an H, monochloro-, dichloro, or trichloro-methyl substituent. (b) In the present model, HPTS is replaced by phenol and excited-state HPTS by tricyanophenol. The cyano substituents are represented by H atoms and the electron-withdrawing effect is described by an external dipolar field. The acid-base character of proton donor and acceptor molecules is controlled by the direction and the magnitude of the external dipolar field. (c) The present model also allows one to study the proton transfer between a strong carboxylic acid and the phenolate ion, a process that has not yet been investigated experimentally by ultrafast pump-probe techniques.
Computational details of the DF/QM model for 2,4,6 tricyanophenol. Model 1 treats all cyano groups as DF/QM substituents, while model 2 includes a full-QM representation of the ortho-substituents. The dipolar fields are created by placing two charges of opposite sign and equal magnitude along three (model 1) or one (model 2) C−H axis. These charges are separated by a fixed distance 0.25 a.u. and the distance between the positive charge and the H atom is constrained to be . The distance between the C and H atoms facing the dipole is also fixed, at a value .
The first-principles alchemy free energy perturbation thermodynamic cycle used to compute the pKa of full-QM 2,6 dicyanophenol, DCN-OH. The DF/QM model 2 (see Fig. 3) is utilized, wherein -DCN-OH corresponds to our model of 2,4,6 tricyanophenol (see text). The remaining acids correspond to 2,6 dicyanophenol molecules in the presence of external dipolar fields of intermediate intensities located in the para-position. , , , and stand for external dipole moments.
The external dipole moment representing a cyano group stabilizes the negative charge on the phenolate oxygen atom via through-space dipole-charge interactions as depicted in (a). In addition, it stabilizes resonance forms as the one depicted in (b) whereby the negative charge is localized on a carbon atom. The acid form is stabilized via dipole-dipole interactions especially when the OH bond is in a “cis” orientation with respect to the external dipole [(c) and (d)].
Time evolution of the distance r between the phenol oxygen atom and the oxygen atom carrying the proton defect (see text and Eq. (6)). The two trajectories correspond to model 1 (bottom panel) and model 2 (top panel). A distance indicates that the covalent O−H bond in tricyanophenol is unbroken and defines the unionized, or covalent state. The red curve at the bottom indicates whether the acid is found in a dissociated or an undissociated state. The definitions of the unionized, dissociated and undissociated states are given in the text.
Time evolution of computed free energy values defined in Eq. (4) (see also text). Free energy values were computed utilizing Bennett’s acceptance ratio approach (see text) by utilizing the first “t” picoseconds from MD trajectories of acids and their conjugate bases.
Oxygen-hydrogen radial distribution function g and coordination number N. The O atom is that of the phenol molecule while H stands for any hydrogen atom from the system. Data was computed for the covalent form of tricyanophenol (blue), phenol (green), tricyanophenolate (black) and phenolate (red). Solid lines correspond to full-QM calculations. The dashed lines correspond to results obtained via model 2, while the triangles correspond to model 1. Full-QM and model 1 are identical (see also text) for phenol and phenolate systems
Oxygen-hydrogen radial distribution function and distance-dependent coordination number for covalently intact trifluoroacetic acid (blue), acetic acid (green), trifluoroacetate (black), and acetate (red). Solid lines correspond to full-QM calculations while dashed lines were obtained using the DF/QM model from Ref. 39. is defined in Eq. (9) and its calculation is depicted in the inset (see also text) and Ref. 63.
Gas-phase proton dissociation energies (kcal/mol) defined in Eq. (5) and total dipole moments (debye) computed using full-QM and DF/QM approaches. The molecules are substituted phenols. The cis and trans isomers are with respect to the relative orientation of the hydroxyl and cyano substituents (see also Fig. 5).
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