  Charmonium sum rules for pseudoscalar 0^{-+} state eta_c(1S) are analyzed
within perturbative QCD and Operator Product Expansion. The perturbative part
of the pseudoscalar correlator is considered at alpha_s^2 order and the
contribution of the gluon condensate <G^2> is taken into account with alpha_s
correction. The OPE series includes the operators of dimension D=6,8 computed
both in the instanton and factorization model. The method of moments in MS-bar
scheme allows to establish acceptable values of the charm quark mass and gluon
condensate, using the experimental mass of eta_c. In result of the analisys the
charm quark mass is found to be m_c(m_c)=1.26+-0.02 GeV independently of the
condensate value. The sensitivity of the results to various approximations for
the massive 3-loop pseudoscalar correlator is discussed.
