We address a central question in the realm of the dynamics of high-n (
= 40-250) Rydberg states of diatomics and large molecules. What is the
coupling responsible for the 'global' l mixing, which results in the
breakdown of the n(3) scaling law for the non-radiative lifetimes and
for the lifetime lengthening (by two to four orders of magnitude) of t
hese states? To explore the implications of intramolecular interaction
s on l mixing and on electronic-rotational energy exchange we analysed
the intramolecular couplings of the ion core dipole, quadrupole and (
anisotropic) polarizability with a non-penetrating (l greater than or
equal to 3) Rydberg electron, in conjunction with the energy gaps betw
een proximal pairs of energy levels. Calculations of the energy gaps a
nd the couplings were performed for the high-n non-penetrating Rydberg
states of NO and for model 'light' (B = 19 cm(-1)) and 'heavy' (B = 0
.05 cm(-1)) polar molecules. All the intramolecular interactions are o
f the form of a power law proportional to l(-n), with eta being determ
ined by the nature of the long-range coupling, by the l dependence of
the quantum defects for multipole couplings and by the I dependence of
angular integrals. We established a bottleneck effect for the intramo
lecular couplings between non-penetrating (l greater than or equal to
3) states. For n- and N+-changing dipole, quadrupole and polarizabilit
y interactions the energetics of the proximal pairs of levels, in conj
unction with the bottleneck effect, prohibit the \n, l, N+, N]-\n', l'
, N+', N], l (greater than or equal to 3) couplings (with n not equal
n') and the electronic-rotational energy exchange. For n- and N+-conse
rving quadrupole and polarizability interactions, the l (greater than
or equal to 3) mixing (which prevails only for l less than or equal to
N+ + N) is also prohibited by the bottleneck effect. 'Global' intramo
lecular l mixing (with both n not equal n' and n = n') in diatomics an
d in large molecules is precluded, implying that the dramatic lengthen
ing of the non-radiative lifetimes of high-n Rydberg states can be ind
uced only by exterior electric held coupling.