  Electric interactions have a strong impact on the structure and dynamics of
biomolecules in their native water environment. Given the variety of water
arrangements in hydration shells and the femto- to subnanosecond time range of
structural fluctuations, there is a strong quest for sensitive noninvasive
probes of local electric fields. The stretching vibrations of phosphate groups,
in particular the asymmetric (PO2)- stretching vibration {\nu}AS(PO2)-, allow
for a quantitative mapping of dynamic electric fields in aqueous environments
via a field-induced redshift of their transition frequencies and concomitant
changes of vibrational line shapes. We present a systematic study of
{\nu}AS(PO2)- excitations in molecular systems of increasing complexity,
including dimethyl phosphate (DMP), short DNA and RNA duplex structures, and
transfer RNA (tRNA) in water. A combination of linear infrared absorption,
two-dimensional infrared (2D-IR) spectroscopy, and molecular dynamics (MD)
simulations gives quantitative insight in electric-field tuning rates of
vibrational frequencies, electric field and fluctuation amplitudes, and
molecular interaction geometries. Beyond neat water environments, the formation
of contact ion pairs of phosphate groups with Mg2+ ions is demonstrated via
frequency upshifts of the {\nu}AS(PO2)- vibration, resulting in a distinct
vibrational band. The frequency positions of contact geometries are determined
by an interplay of attractive electric and repulsive exchange interactions.
