We present the first generation and spectroscopic identification of the higher-lying E conformer of the simplest aromatic carboxylic acid, benzoic acid (la), as its O-deuterated isotopologue (E)-d(1)-1a using matrix-isolation techniques; the parent (E)-1a could not be observed because of fast H-tunneling to the more stable conformer (Z)-1a. Even deuterated (E)-d(1)-1a converts quickly back to (Z)-d(1)-1a through D-tunneling with a half-life (tau) of similar to 12 min in Ar at 11 K. Tunneling computations using an Eckert barrier in conjunction with a CCSD(T)/cc-pVTZ//MP2/cc-pVDZ + ZPVE intrinsic reaction path revealed that tau of (E)-1a is only similar to 10(-5) min, in marked contrast to those of simple aliphatic acids, which are in the range of minutes. The electronic substituent effects on D-tunneling in para-substituted benzoic acid derivatives (p-X-PhCOOD, d(1)-1) were systematically studied in Ar matrices at 11 K to derive the first Hammett relationships for atom tunneling, sigma-Electron donors (X = alkyl) increase the half-life of dr(1)-1, while sigma-acceptor/pi-donor groups (X = OD, NH2, halogen) and to an even greater extent a sigma-/pi-acceptor group (X = NO2) decrease tau. The latter finding is in line with the smaller E-to-Z reaction barriers and narrower reaction widths for the isomerization. Tunneling substituent constants (male) for this conformational isomerization were derived experimentally and computationally.