Does absentmindedness during the opening "Allahu Akbar" (Takbirat al-Ihram) affect the validity of the prayer?

What the questioner is experiencing is waswasah (whispers/obsessive thoughts). The takbirat al-ihram (opening takbir) consists of only two words, "Allahu Akbar" (Allah is Greater), and they are too short for the mind to wander during their pronouncement. To overcome waswasah, the following is recommended: seeking Allah's help and consistently supplicating to Him, frequently remembering Him (dhikr), always seeking refuge with Allah from Satan, ignoring the waswasah and not dwelling on it, and striving to maintain presence of mind and thought during prayer.