A patient ingested some unknown substance and presented with myoclonic jerks seizures, tachycardia and hypotension.The ECG showed a hea rate of 120/minute with QRS interval of 0.16 seconds. The atrial blood revealed a ph of 7.25, PCO2 of 30 mmHg and HCO3 of 15 mmol/L. the most likely cause of poisoning is ingesion of-